Question

In a class of students, the following data table summarizes how many students have a brother or a sister. What is the probability that a student has a brother given that they have a sister?
Has a brother Does not have a brother
Has a sister 7 2
Does not have a sister 3 14

Answer 1

P(has brother | has sister) = number with both / number with sister = 7 / (7+2) = 7/9 ≈ 0.7778 ≈ 77.78%.