Question

What’s the correct answer for this?

Answer 1

1/4

Explanation:

Let's denote the probabilities as following:

Probability that a teenager has a sister:

P(A) = 12/28

Probability that a teenager has a brother:

P(B) = 7/28

Probability that a teenager has both a sister and a brother:

P(A⋂B) = 3/28

Probability that a selected teenager has a sister also has a brother, or in other words, he/she has a brother, given he/she had a sister:

P(B|A)

Let's apply the formula of conditional probability to work out P(B|A)

P(B|A) = P(A⋂B)/P(A) = (3/28)/(12/28) = (3*28)/(12*28) = 3/12 = 1/4

=> Option C is correct

Hope this helps!