Question

In a class 26 of students, 17 have a brother and 7 have a sister. There are 4 students who do not have any siblings. What is the probability that a student has a brother given that they have a sister

Answer 1

Number of students = 26
Number of students without siblings = 4 
⇒ Number of students with at least one siblings = 26 - 4 = 22
 
Number of students with brothers = 17
Number of students with sisters = 7
⇒ Number of students with both a brother and a sister = 17 + 7 - 22 = 2

P(student has a brother, given that they have a sister) = 2/7

Answer: 2/7