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Your friend claims that the probability of \(A\) given \(B\) is the same as the probability of \(B\) given \(A\). How would you convince him that he is wrong?

Short Answer

Expert verified
To show that the probability of \(A\) given \(B\) is not the same as the probability of \(B\) given \(A\), we can use an example with a deck of playing cards. Let event \(A\) be getting a king and event \(B\) be getting a red card. We find that \(P(A|B) = \frac{1}{13}\) and \(P(B|A) = \frac{1}{2}\), which are not equal. Moreover, we can look at the formulas for each conditional probability, \(P(A|B) = \frac{P(A \cap B)}{P(B)}\) and \(P(B|A) = \frac{P(A \cap B)}{P(A)}\). The denominators are different, indicating that these probabilities are generally not the same.

Step by step solution

01

Clarify the difference between the two conditional probabilities

The probability of \(A\) given \(B\), denoted as \(P(A|B)\), is the probability of event \(A\) happening when we know that event \(B\) has happened. On the other hand, the probability of \(B\) given \(A\), denoted as \(P(B|A)\), is the probability of event \(B\) happening when we know that event \(A\) has happened.
02

Provide a simple example to illustrate the difference

Let's consider a scenario where we have a deck of playing cards. Suppose event \(A\) is getting a king and event \(B\) is getting a red card. We want to find the probabilities of \(A\) given \(B\) and \(B\) given \(A\). - \(P(A|B)\) There are 26 red cards in a deck of 52 playing cards. Out of these 26 red cards, only 2 are kings. Therefore, the probability of getting a king, given that we have a red card, is: \[P(A|B) = \frac{2}{26} = \frac{1}{13}\] - \(P(B|A)\) There are 4 kings in a deck of 52 playing cards. Out of these 4 kings, 2 are red cards. Therefore, the probability of getting a red card, given that we have a king, is: \[P(B|A) = \frac{2}{4} = \frac{1}{2}\] Since \(\frac{1}{13} \neq \frac{1}{2}\), we can conclude that \(P(A|B)\) is not the same as \(P(B|A)\).
03

Show the formulas for each conditional probability

We can also show the formulas for each conditional probability, which are as follows: \[P(A|B) = \frac{P(A \cap B)}{P(B)}\] \[P(B|A) = \frac{P(A \cap B)}{P(A)}\] As we can see, the denominators for each formula are different (\(P(B)\) and \(P(A)\)), which indicates that in general, the two conditional probabilities are not the same. This further supports our argument that the probability of \(A\) given \(B\) is not the same as the probability of \(B\) given \(A\).

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