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The example used to introduce conditional probability described a family with two children each of whom was equally likely to be a boy or a girl. The example showed that if it is known that one child is a boy, the probability that the other child is a boy is \(1 / 3\). Now imagine the same kind of family- two children each of whom is equally likely to be a boy or a girl. Suppose you meet one of the children and sce that it is a boy. What is the probability that the other child is a boy? Explain. (Be careful. The answer may surprise you.)

Short Answer

Expert verified
The probability that the other child is a boy, given that we see one of the children is a boy, is \(\frac{1}{3}\).

Step by step solution

01

Construct a sample space

First, let's construct the sample space for the possible genders of the two children. We can represent each child's gender using the letters "B" for boys and "G" for girls. The sample space consists of all possible combinations of two children: 1. Boy, Boy (BB) 2. Boy, Girl (BG) 3. Girl, Boy (GB) 4. Girl, Girl (GG)
02

Determine the event A: We see one child is a boy

We are given that we see one of the children, and the child is a boy. This could happen in two ways: either the older child is a boy (BG or BB) or the younger child is a boy (GB or BB). So the event A consists of the combinations (BB), (BG), and (GB).
03

Determine the event B: The other child is a boy

In this case we are interested in the combinations where the other child is a boy. So, the event B consists of the combination (BB).
04

Find the probability of event A

Since there are three possible combinations in event A (BB, BG, and GB) and four possible combinations in the sample space, the probability of event A is \( \frac{3}{4} \).
05

Find the probability of A and B

Event A and B can happen together only if both children are boys (BB). Since there is one combination (BB) out of the four possible combinations in the sample space, the probability of A and B is \( \frac{1}{4} \).
06

Calculate the conditional probability of B given A

Now we can use the conditional probability formula to find the desired probability: P(B|A) = P(A and B) / P(A) = \(\frac{1/4}{3/4}\) = \(\frac{1}{3}\).
07

Interpretation of the result

The probability that the other child is a boy, given that we see one of the children is a boy, is \(\frac{1}{3}\). This might be surprising because it's different from the \( \frac{1}{2} \) chance we would expect if we knew nothing about the family. However, this result is due to the fact that we are given some information about one of the children and not considering all equally likely scenarios.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Sample Space
In probability theory, the sample space is the entire set of possible outcomes of a random experiment. To effectively solve probability problems, we must first identify all the possible outcomes. In the context of the provided exercise, we consider a family with two children. Each child can independently be a boy (B) or a girl (G), leading to the sample space represented as {BB, BG, GB, GG}.

Understanding and listing the sample space is crucial for the correct calculation of probabilities. It guarantees that all possible outcomes are included in our analysis, ensuring we do not overlook any scenarios that could affect the result. The exercise improvement advice would emphasize the importance of clearly defining the sample space before calculating any probabilities, as omitting even one possible outcome can significantly distort the final probability estimate.
Probability Theory
The exercise on conditional probability is rooted in probability theory, which is a branch of mathematics concerned with the analysis of random phenomena. The main goal is to determine the likelihood of a particular event occurring within a defined set of possibilities, or sample space. In our example, probability theory guides us to calculate the chance of the second child being a boy, given that we've seen one child who is a boy.

When developing textbook solutions, it's important to illustrate how probability theory principles apply to practical situations, like the classic children's gender problem. The advice to enhance this exercise would include ensuring that students understand how to use the formula for conditional probability and how the provision of additional information (one child being a boy) changes the calculation compared to the unconditional scenarios.
Independent Events
Two events are said to be independent if the occurrence of one does not affect the probability of the occurrence of the other. However, the exercise at hand deals with a non-independent scenario, as the probability of the second child being a boy is influenced by the knowledge that one child is already a boy. The exercise shows that such information leads to the concept of conditional probability, which differs from the probability assessment of independent events.

To better understand the notion of independent events, contrasting them with the current problem can be helpful. If we did not know the gender of one child, each child's gender would be an independent event, with the probability of being a boy or girl being 1/2 for each child separately. An improvement tip for the exercise would be to include a comparison of conditional versus independent probabilities, to underscore the influence of given information on the calculation of probabilities.

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