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 sister given that they have a brother?

Question

Has a brother	Does not have a brother
Has a sister	7	12
Does not have a sister	3	5

Answer 1

if you move the two titles in the top row over to the right one square, then the conditional probability that someone with a brother also has a sister = 70% = 7/(7+3) = 7/10 = .70

only other alternative would be to move that title in the left top to the right top

then the probabillity = 12/(12+5) = 12/17= about 71%

either way, the conditional probability of having a sister if the student has a brother = about 70%