14.1-Probability- A Theoretical Approach
14.1-Probability- A Theoretical Approach Important Formulae
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Grade 10 → Math → Probability → 14.1-Probability- A Theoretical Approach
- Differentiate between Empirical probability and theoretical probability in order to find the two for a variety of cases.
- Calculate the probability of given events in an experiment in order to comment whether they are complementary events/Sure event/impossible event.
- Represent using organized lists, tables, or tree diagrams in order to List the sample space for compound events.
- Calculate the probability of various events in order to rank them from most to least probable.
Probability is a branch of mathematics that deals with the likelihood or chance of different outcomes. In this theoretical approach, we focus on defining probability, calculating it, and understanding its applications. Probability helps us make informed predictions and decisions based on the analysis of random events.
1. Definition of Probability
Probability is defined as the measure of the likelihood that an event will occur. It is expressed as a number between 0 and 1, where:
- A probability of 0 indicates that the event will not occur.
- A probability of 1 indicates that the event will certainly occur.
- A probability of 0.5 indicates an equal chance of the event occurring or not occurring.
2. Mathematical Representation
The probability of an event $A$ can be mathematically represented as:
$$P(A) = \frac{\text{Number of favorable outcomes}}{\text{Total number of outcomes}}$$
3. Types of Events
In probability, events can be classified into various categories:
- Sure Event: An event that is certain to occur. For example, when rolling a die, the event of getting a number between 1 and 6 is a sure event.
- Impossible Event: An event that cannot occur. For instance, getting a number greater than 6 when rolling a die.
- Complementary Event: The complement of an event $A$, denoted as $A'$, consists of all outcomes that are not in $A$. The relationship is given by:
- Mutually Exclusive Events: Two events are mutually exclusive if they cannot occur simultaneously. For example, when flipping a coin, getting heads and tails at the same time is impossible.
$$P(A) + P(A') = 1$$
4. Sample Space
The sample space, denoted as $S$, is the set of all possible outcomes of an experiment. For example, when rolling a six-sided die, the sample space is:
$$S = \{1, 2, 3, 4, 5, 6\}$$
5. Examples of Calculating Probability
Let’s consider some practical examples to understand probability calculations:
- Example 1: What is the probability of rolling a 4 on a die?
- Example 2: What is the probability of rolling an even number on a die?
The number of favorable outcomes is 1 (rolling a 4) and the total number of outcomes is 6. Thus:
$$P(\text{rolling a 4}) = \frac{1}{6}$$
The favorable outcomes are $\{2, 4, 6\}$, so there are 3 favorable outcomes:
$$P(\text{rolling an even number}) = \frac{3}{6} = \frac{1}{2}$$
6. Theoretical Probability vs Experimental Probability
Theoretical probability is based on the possible outcomes in a perfect world. In contrast, experimental probability is based on actual experiments and observations. The formula for experimental probability is:
$$P(E) = \frac{\text{Number of times event E occurs}}{\text{Total number of trials}}$$
7. Law of Large Numbers
The Law of Large Numbers states that as the number of trials increases, the experimental probability of an event will get closer to its theoretical probability. This principle underlines the importance of large samples in probability theory.
8. Applications of Probability
Probability has numerous applications in various fields, including:
- Statistics: Used for data analysis and interpretation.
- Finance: Helps in assessing risks and making investment decisions.
- Games and Sports: Used to determine odds and strategies.
- Science and Engineering: Applied in quality control and reliability testing.
Understanding probability is essential for making informed decisions based on uncertainty and for analyzing random phenomena in various real-world scenarios.
Picture of six sided dice.
Diacritica, CC BY-SA 3.0, via Wikimedia Commons
Solved Example: 14-1-01
Complete the following statements:
(i) Probability of an event E + Probability of the event ‘not E’ = ______ .
(ii) The probability of an event that cannot happen is called ______ .
(iii) The probability of an event that is certain to happen is called ______ .
(iv) The sum of the probabilities of all the elementary events of an experiment is ______ .
(v) The probability of an event is greater than or equal to ______ and less than or equal to ______ .
Solution:
Complete the Following Statements
(i) Probability of an event E + Probability of the event ‘not E’ = 1.
(ii) The probability of an event that cannot happen is called 0.
(iii) The probability of an event that is certain to happen is called 1.
(iv) The sum of the probabilities of all the elementary events of an experiment is 1.
(v) The probability of an event is greater than or equal to 0 and less than or equal to 1.
Solved Example: 14-1-02
Which of the following experiments have equally likely outcomes? Explain.
(i) A driver attempts to start a car. The car starts or does not start.
(ii) A player attempts to shoot a basketball. She/he shoots or misses the shot.
(iii) A trial is made to answer a true-false question. The answer is right or wrong.
(iv) A baby is born. It is a boy or a girl.
Solution:
Equally Likely Outcomes in Experiments
(i) A driver attempts to start a car. The car starts or does not start.
Explanation: This experiment has two outcomes that may not be equally likely, as the car may have a higher chance of starting if it is in good condition.
(ii) A player attempts to shoot a basketball. She/he shoots or misses the shot.
Explanation: This experiment may not have equally likely outcomes, as a player's skill affects the likelihood of making the shot.
(iii) A trial is made to answer a true-false question. The answer is right or wrong.
Explanation: This experiment may have equally likely outcomes if the question is random and does not favor either answer.
(iv) A baby is born. It is a boy or a girl.
Explanation: This experiment is often considered to have equally likely outcomes, assuming equal probability for boys and girls at birth.
Solved Example: 14-1-03
Why is tossing a coin considered to be a fair way of deciding which team should get the ball at the beginning of a football game?
Solution:
Tossing a coin is considered fair because each outcome—heads or tails—has an equal probability of 50%. This randomness ensures that neither team has an advantage, allowing for an impartial decision to determine possession.
Solved Example: 14-1-04
Which of the following cannot be the probability of an event?
(A) $\dfrac{2}{3}$
(B) –1.5
(C) 15%
(D) 0.7
Solution:
Probability of an event :
\[0 \leq P(E) \leq 1\]
(B) –1.5 cannot be the probability of an event, as probabilities must be between 0 and 1, both extreme values inclusive.
Solved Example: 14-1-05
If P(E) = 0.05, what is the probability of ‘not E’?
Solution:
The probability of "not E," often denoted as \( P(\text{not } E) \), can be calculated using the formula:
\[
P(\text{not } E) = 1 - P(E)
\]
Given \( P(E) = 0.05 \):
\[
P(\text{not } E) = 1 - 0.05 = 0.95
\]
So, the probability of "not E" is 0.95.
Solved Example: 14-1-06
A bag contains lemon flavoured candies only. Malini takes out one candy without looking into the bag. What is the probability that she takes out:
(i) An orange flavoured candy?
(ii) A lemon flavoured candy?
Solution:
(i) The probability that Malini takes out an orange flavoured candy is 0, since the bag contains only lemon flavoured candies.
(ii) The probability that she takes out a lemon flavoured candy is 1, as that is the only type in the bag.
Solved Example: 14-1-07
It is given that in a group of 3 students, the probability of 2 students not having the same birthday is 0.992. What is the probability that the 2 students have the same birthday?
Solution:
The probability that the two students have the same birthday can be found by subtracting the probability of them not having the same birthday from 1:
\[
P(\text{same birthday}) = 1 - P(\text{not same birthday}) = 1 - 0.992 = 0.008
\]
So, the probability is 0.008.
Solved Example: 14-1-08
A bag contains 3 red balls and 5 black balls. A ball is drawn at random from the bag. What is the probability that the ball drawn is:
(i) Red?
(ii) Not red?
Solution:
To find the probabilities, we first determine the total number of balls in the bag:
Total balls = 3 (red) + 5 (black) = 8 balls.
(i) Probability of drawing a red ball:
\[P(\text{red}) = \frac{\text{Number of red balls}}{\text{Total number of balls}} = \dfrac{3}{8}\]
(ii) Probability of drawing a ball that is not red:
\[P(\text{not red}) = 1 - P(\text{red}) = 1 - \dfrac{3}{8} = \dfrac{5}{8}\]
So, the probabilities are:
(i) \( \dfrac{3}{8} \)
(ii) \( \dfrac{5}{8} \)
Solved Example: 14-1-09
A box contains 5 red marbles, 8 white marbles and 4 green marbles. One marble is taken out of the box at random. What is the probability that the marble taken out will be (i) red ? (ii) white ? (iii) not green?
Solution:
To find the probabilities, we first calculate the total number of marbles:
Total marbles = 5 (red) + 8 (white) + 4 (green) = 17.
(i) Probability of drawing a red marble:
\[ P(\text{red}) = \frac{5}{17} \]
(ii) Probability of drawing a white marble:
\[ P(\text{white}) = \frac{8}{17} \]
(iii) Probability of not drawing a green marble:
Total non-green marbles = 5 (red) + 8 (white) = 13.
\[ P(\text{not green}) = \frac{13}{17} \]
Solved Example: 14-1-10
A piggy bank contains hundred 50p coins, fifty Re. 1 coins, twenty Rs. 2 coins and ten Rs. 5 coins. If it is equally likely that one of the coins will fall out when the bank is turned upside down,what is the probability tha tthe coin:
(i) Will be a 50p coin?
(ii) Will not be a Rs. 5 coin?
Solution:
To find the probabilities, we first calculate the total number of coins in the piggy bank.
Total number of coins:
- 50p coins: 100
- Re. 1 coins: 50
- Rs. 2 coins: 20
- Rs. 5 coins: 10
Total coins = 100 + 50 + 20 + 10 = 180.
(i) Probability of drawing a 50p coin:
\[ P(\text{50p coin}) = \frac{100}{180} = \frac{5}{9} \]
(ii) Probability of not drawing a Rs. 5 coin:
Total coins that are not Rs. 5 = 100 (50p) + 50 (Re. 1) + 20 (Rs. 2) = 170.
\[ P(\text{not Rs. 5 coin}) = \frac{170}{180} = \frac{17}{18} \]
Solved Example: 14-1-11
Gopi buys a fish from a shop for his aquarium. The shopkeeper takes out one fish at random from a tank containing 5 male fish and 8 female fish (see Fig. 14.4). What is the probability that the fish taken out is a male fish?
Solution:
To find the probability that the fish taken out is a male fish, we first determine the total number of fish in the tank.
Total fish = 5 male fish + 8 female fish = 13 fish.
Now, the probability of drawing a male fish is given by the ratio of the number of male fish to the total number of fish:
\[ P(\text{male fish}) = \frac{\text{Number of male fish}}{\text{Total number of fish}} = \dfrac{5}{13} \]
So, the probability that the fish taken out is a male fish is \( \dfrac{5}{13} \).
Solved Example: 14-1-12
A game of chance consists of spinning an arrow which comes to rest pointing at one of the numbers 1, 2, 3, 4, 5, 6, 7, 8 (see Fig. 14.5 ), and these are equally likely outcomes. What is the probability that it will point at:
(i) 8?
(ii) An odd number?
(iii) A number greater than 2?
(iv) A number less than 9?
Solution:
In this game of chance, the total number of possible outcomes when the arrow is spun is 8 (the numbers 1 through 8).
(i) Probability that it will point at 8:
\[ P(8) = \dfrac{1}{8} \]
(ii) Probability that it will point at an odd number (1, 3, 5, 7):
There are 4 odd numbers.
\[ P(\text{odd number}) = \dfrac{4}{8} = \frac{1}{2} \]
(iii) Probability that it will point at a number greater than 2 (3, 4, 5, 6, 7, 8):
There are 6 numbers greater than 2.
\[ P(\text{greater than 2}) = \dfrac{6}{8} = \dfrac{3}{4} \]
(iv) Probability that it will point at a number less than 9:
All numbers (1, 2, 3, 4, 5, 6, 7, 8) are less than 9, so there are 8 favorable outcomes.
\[ P(\text{less than 9}) = \dfrac{8}{8} = 1 \]
To summarize:
(i) \( \dfrac{1}{8} \)
(ii) \( \dfrac{1}{2} \)
(iii) \( \dfrac{3}{4} \)
(iv) \( 1 \)
Solved Example: 14-1-13
A die is thrown once. Find the probability of getting:
(i) A prime number;
(ii) A number lying between 2 and 6;
(iii) An odd number.
Solution:
When a die is thrown, the possible outcomes are the numbers 1, 2, 3, 4, 5, and 6. This gives us a total of 6 outcomes.
(i) **Probability of getting a prime number (2, 3, 5):**
The prime numbers on a die are 2, 3, and 5, which gives us 3 favorable outcomes.
\[
P(\text{prime number}) = \frac{3}{6} = \frac{1}{2}
\]
(ii) **Probability of getting a number lying between 2 and 6 (3, 4, 5):**
The numbers that lie between 2 and 6 are 3, 4, and 5, giving us 3 favorable outcomes.
\[
P(\text{between 2 and 6}) = \frac{3}{6} = \frac{1}{2}
\]
(iii) **Probability of getting an odd number (1, 3, 5):**
The odd numbers on a die are 1, 3, and 5, giving us 3 favorable outcomes.
\[
P(\text{odd number}) = \frac{3}{6} = \frac{1}{2}
\]
To summarize:
- (i) \( \frac{1}{2} \)
- (ii) \( \frac{1}{2} \)
- (iii) \( \frac{1}{2} \)
Solved Example: 14-1-14
One card is drawn from a well-shuffled deck of 52 cards. Find the probability of getting:
(i) A king of red colour
(ii) A face card
(iii) A red face card
(iv) The jack of hearts
(v) A spade
(vi) The queen of diamonds
Solution:
In a standard deck of 52 cards, we can calculate the probabilities for each scenario:
(i) Probability of getting a king of red color (2 cards: King of Hearts, King of Diamonds):
\[ P(\text{red king}) = \dfrac{2}{52} = \dfrac{1}{26} \]
(ii) Probability of getting a face card (12 cards: Kings, Queens, Jacks of all suits):
\[ P(\text{face card}) = \dfrac{12}{52} = \dfrac{3}{13} \]
(iii) Probability of getting a red face card (6 cards: King, Queen, Jack of Hearts and Diamonds):
\[ P(\text{red face card}) = \dfrac{6}{52} = \dfrac{3}{26} \]
(iv) Probability of getting the jack of hearts (1 card):
\[ P(\text{jack of hearts}) = \dfrac{1}{52} \]
(v) Probability of getting a spade (13 cards):
\[ P(\text{spade}) = \dfrac{13}{52} = \dfrac{1}{4}\]
(vi) Probability of getting the queen of diamonds (1 card):
\[ P(\text{queen of diamonds}) = \dfrac{1}{52}\]
To summarize:
(i) \( \dfrac{1}{26} \)
(ii) \( \dfrac{3}{13} \)
(iii) \( \dfrac{3}{26} \)
(iv) \( \dfrac{1}{52} \)
(v) \( \dfrac{1}{4} \)
(vi) \( \dfrac{1}{52} \)
Solved Example: 14-1-15
Five cards—the ten, jack, queen, king and ace of diamonds, are well-shuffled with their face downwards. One card is then picked up at random.
(i) What is the probability that the card is the queen?
(ii) If the queen is drawn and put aside, what is the probability that the second card
picked up is (a) an ace? (b) a queen?
Solution:
In this scenario, there are 5 cards: the ten, jack, queen, king, and ace of diamonds.
(i) **Probability that the card is the queen:**
There is 1 queen among the 5 cards.
\[
P(\text{queen}) = \frac{1}{5}
\]
(ii) If the queen is drawn and put aside, there are now 4 cards remaining (ten, jack, king, and ace).
(a) **Probability that the second card picked up is an ace:**
There is 1 ace among the 4 remaining cards.
\[
P(\text{ace}) = \frac{1}{4}
\]
(b) **Probability that the second card picked up is a queen:**
There are no queens left since the queen was put aside.
\[
P(\text{queen}) = 0
\]
To summarize:
- (i) \( \frac{1}{5} \)
- (ii)(a) \( \frac{1}{4} \)
- (ii)(b) \( 0 \)
Solved Example: 14-1-16
12 defective pens are accidentally mixed with 132 good ones. It is not possible to just look at a pen and tell whether or not it is defective. One pen is taken out at random from this lot. Determine the probability that the pen taken out is a good one.
Solution:
To determine the probability that a pen taken out at random is a good one, we first need to find the total number of pens.
Total pens = Number of good pens + Number of defective pens
Total pens = 132 good pens + 12 defective pens = 144 pens.
Now, the probability of picking a good pen is given by the ratio of the number of good pens to the total number of pens:
\[
P(\text{good pen}) = \frac{\text{Number of good pens}}{\text{Total number of pens}} = \frac{132}{144} = \frac{11}{12}.
\]
Thus, the probability that the pen taken out is a good one is \( \frac{11}{12} \).
Solved Example: 14-1-17
(i) A lot of 20 bulbs contain 4 defective ones. One bulb is drawn at random from the lot. What is the probability that this bulb is defective?
(ii) Suppose the bulb drawn in (i) is not defective and is not replaced. Now one bulb is drawn at random from the rest. What is the probability that this bulb is not defective ?
Solution:
(i) To find the probability that a bulb drawn is defective, we use the number of defective bulbs and the total number of bulbs.
Total bulbs = 20
Defective bulbs = 4
\[
P(\text{defective}) = \frac{\text{Number of defective bulbs}}{\text{Total number of bulbs}} = \frac{4}{20} = \frac{1}{5}.
\]
(ii) If the first bulb drawn is not defective, then there are now 19 bulbs left (16 good and 3 defective).
Now, the probability that the next bulb drawn is not defective is:
\[
P(\text{not defective}) = \frac{\text{Number of good bulbs}}{\text{Total remaining bulbs}} = \frac{16}{19}.
\]
To summarize:
- (i) \( \frac{1}{5} \)
- (ii) \( \frac{16}{19} \)
Solved Example: 14-1-18
A box contains 90 discs which are numbered from 1 to 90. If one disc is drawn at random from the box, find the probability that it bears:
(i) A two-digit number
(ii) A perfect square number
(iii) A number divisible by 5.
Solution:
To find the probabilities, we start with the total number of discs, which is 90.
(i) **Probability of drawing a two-digit number:**
The two-digit numbers are from 10 to 90 (inclusive).
Count of two-digit numbers = 90 - 10 + 1 = 81.
\[
P(\text{two-digit number}) = \frac{81}{90} = \frac{9}{10}.
\]
(ii) **Probability of drawing a perfect square number:**
The perfect squares between 1 and 90 are: 1, 4, 9, 16, 25, 36, 49, 64, 81 (which are \(1^2\) to \(9^2\)).
Count of perfect squares = 9.
\[
P(\text{perfect square}) = \frac{9}{90} = \frac{1}{10}.
\]
(iii) **Probability of drawing a number divisible by 5:**
The numbers divisible by 5 from 1 to 90 are: 5, 10, 15, ..., 90.
These form an arithmetic sequence where \(a = 5\), \(d = 5\), and the last term is 90.
To find the count:
\[
n = \frac{90 - 5}{5} + 1 = 18.
\]
\[
P(\text{divisible by 5}) = \frac{18}{90} = \frac{1}{5}.
\]
To summarize:
- (i) \( \frac{9}{10} \)
- (ii) \( \frac{1}{10} \)
- (iii) \( \frac{1}{5} \)
Solved Example: 14-1-19
A child has a die whose six faces show the letters as given below:
| A | B | C | D | E | A |
The die is thrown once. What is the probability of getting (i) A? (ii) D?
Solution:
The die has the following faces: A, B, C, D, E, A. This gives us a total of 6 faces, with A appearing twice.
(i) Probability of getting A:
There are 2 faces showing A.
\[P(A) = \dfrac{\text{Number of A faces}}{\text{Total faces}} = \dfrac{2}{6} = \dfrac{1}{3}\]
(ii) Probability of getting D:
There is 1 face showing D.
\[P(D) = \dfrac{\text{Number of D faces}}{\text{Total faces}} = \dfrac{1}{6}\]
To summarize:
(i) \( \dfrac{1}{3} \)
(ii) \( \dfrac{1}{6} \)
Solved Example: 14-1-20
Suppose you drop a die at random on the rectangular region shown in Fig. 14.6. What is the probability that it will land inside the circle with diameter 1m?
Solution:
To determine the probability that a die lands inside a circle with a diameter of 1 meter, we first need to calculate the area of the circle and the area of the rectangular region in which the die can land.
1. **Area of the circle:**
The radius \( r \) of the circle is half of the diameter:
\( r = \frac{1}{2} \) m.
The area \( A \) of the circle is given by the formula:
\[
A_{\text{circle}} = \pi r^2 = \pi \left(\frac{1}{2}\right)^2 = \frac{\pi}{4} \, \text{m}^2.
\]
2. **Area of the rectangular region:**
If the rectangular region has a width and height both greater than or equal to 1 m (common assumptions), let's assume it is a square with a side length of 1 m for simplicity:
\[
A_{\text{rectangle}} = 1 \times 1 = 1 \, \text{m}^2.
\]
3. **Probability of landing inside the circle:**
The probability \( P \) is the ratio of the area of the circle to the area of the rectangle:
\[
P = \frac{A_{\text{circle}}}{A_{\text{rectangle}}} = \frac{\frac{\pi}{4}}{1} = \frac{\pi}{4} \approx 0.785.
\]
Thus, the probability that the die will land inside the circle is approximately \( \frac{\pi}{4} \) or about 0.785.
Solved Example: 14-1-21
A lot consists of 144 ball pens of which 20 are defective and the others are good. Nuri will buy a pen if it is good, but will not buy if it is defective. The shopkeeper draws one pen at random and gives it to her. What is the probability that:
(i) She will buy it ?
(ii) She will not buy it ?
Solution:
In the lot of 144 ball pens, there are 20 defective pens and \( 144 - 20 = 124 \) good pens.
(i) Probability that Nuri will buy the pen:
Since she will buy a pen only if it is good, the probability is given by the ratio of good pens to the total number of pens:
\[
P(\text{buys}) = \frac{\text{Number of good pens}}{\text{Total number of pens}} = \frac{124}{144} = \frac{31}{36}.
\]
(ii) Probability that she will not buy the pen:
This occurs if the pen is defective. The probability is given by the ratio of defective pens to the total number of pens:
\[
P(\text{does not buy}) = \frac{\text{Number of defective pens}}{\text{Total number of pens}} = \frac{20}{144} = \frac{5}{36}.
\]
To summarize:
- (i) \( \frac{31}{36} \)
- (ii) \( \frac{5}{36} \)
Solved Example: 14-1-22
Refer to Example 13. (i) Complete the following table:
Solved Example: 14-1-23
A game consists of tossing a one rupee coin 3 times and noting its outcome each time. Hanif wins if all the tosses give the same result i.e., three heads or three tails, and loses otherwise. Calculate the probability that Hanif will lose the game.
Solution:
To determine the probability that Hanif loses the game, we first need to find the total number of possible outcomes when tossing a coin 3 times.
1. Total outcomes:
Each toss has 2 possible results (heads or tails). Therefore, for 3 tosses:
\[\text{Total outcomes} = 2^3 = 8.\]
2. Winning outcomes:
Hanif wins if all tosses are the same, which can happen in two ways:
- All heads (HHH)
- All tails (TTT)
Thus, there are 2 winning outcomes.
3. Losing outcomes:
The number of outcomes where Hanif loses is the total outcomes minus the winning outcomes:
\[ \text{Losing outcomes} = \text{Total outcomes} - \text{Winning outcomes} = 8 - 2 = 6.\]
4. Probability of losing:
Now, we can calculate the probability that Hanif loses:
\[ P(\text{loses}) = \frac{\text{Losing outcomes}}{\text{Total outcomes}} = \frac{6}{8} = \frac{3}{4}. \]
Therefore, the probability that Hanif will lose the game is \( \frac{3}{4} \).
Solved Example: 14-1-24
A die is thrown twice. What is the probability that:
(i) 5 will not come up either time?
(ii) 5 will come up at least once?
[Hint : Throwing a die twice and throwing two dice simultaneously are treated as the same experiment]
Solution:
When a die is thrown twice, there are a total of \(6 \times 6 = 36\) possible outcomes.
(i) Probability that 5 will not come up either time:
To find this probability, we first determine the number of outcomes where 5 does not appear. Each die has 5 outcomes that are not 5 (1, 2, 3, 4, 6).
The number of outcomes where 5 does not appear in two throws is:
\[5 \times 5 = 25.\]
Now, the probability is:
\[P(\text{not 5 either time}) = \dfrac{25}{36}\]
(ii) Probability that 5 will come up at least once:
The probability of getting at least one 5 can be found using the complement rule:
\[P(\text{at least one 5}) = 1 - P(\text{not 5 either time}).\]
Using the result from (i):
\[P(\text{at least one 5}) = 1 - \dfrac{25}{36} = \dfrac{11}{36}.\]
To summarize:
(i) \( \dfrac{25}{36} \)
(ii) \( \dfrac{11}{36} \)
Solved Example: 14-1-25
Which of the following arguments are correct and which are not correct? Give reasons for your answer.
(i) If two coins are tossed simultaneously there are three possible outcomes — two heads, two tails or one of each. Therefore, for each of these outcomes, the probability is $\dfrac{1}{3}$
(ii) If a die is thrown, there are two possible outcomes—an odd number or an even
number. Therefore, the probability of getting an odd number is $\dfrac{1}{2}$.
Solution:
Let's evaluate each argument:
(i) Argument: If two coins are tossed simultaneously, there are three possible outcomes — two heads, two tails, or one of each. Therefore, for each of these outcomes, the probability is \(\dfrac{1}{3}\).
Evaluation: This argument is not correct. When tossing two coins, the possible outcomes are:
- HH (two heads)
- HT (one head, one tail)
- TH (one head, one tail)
- TT (two tails)
Thus, there are actually 4 outcomes: HH, HT, TH, and TT. The correct probabilities for each outcome are:
- \(P(\text{two heads}) = \dfrac{1}{4}\)
- \(P(\text{two tails}) = \dfrac{1}{4}\)
- \(P(\text{one of each}) = \dfrac{2}{4} = \dfrac{1}{2}\)
So, the probabilities do not equal \(\dfrac{1}{3}\).
---
(ii) Argument: If a die is thrown, there are two possible outcomes—an odd number or an even number. Therefore, the probability of getting an odd number is \(\dfrac{1}{2}\).
Evaluation: This argument is correct. A standard die has six faces showing the numbers 1, 2, 3, 4, 5, and 6. The odd numbers are 1, 3, and 5 (3 outcomes), while the even numbers are 2, 4, and 6 (3 outcomes). Since there are 3 odd numbers and 3 even numbers, the probabilities are:
- \(P(\text{odd number}) = \dfrac{3}{6} = \dfrac{1}{2}\)
- \(P(\text{even number}) = \dfrac{3}{6} = \dfrac{1}{2}\)
Therefore, the argument is correct.