Answer:
1/3
Step-by-step 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
