Question

In a class of 27 students, 10 have a brother and 12 have a sister. There are 4 students who have a brother and a sister. What is the probability that a student who has a sister also has a brother?

Answer 1

1/3

Explanation:

10 students have a brother.

12 students have a sister.

4 students have a brother and a sister.

6 students have only a brother.

8 students have only a sister.

4 students have a brother and a sister.

Given that a student has a sister, there are 12 such students (8 with only a sister and 4 with a brother and a sister).

Of those 12 with a sister, there are 4 with a brother and a sister, so there are 4 out of 12 with a sister who also have a brother.

p = 4/12 = 1/3