Question

In a class of 23 students, 16 have a brother and 15 have a sister. There are 11 students

who have a brother and a sister. What is the probability that a student chosen
randomly from the class is an only child?

Answer 1

• 3/23

Explanation:

• 11 students have a brother and a sister
• 16 students have a brother and 16 - 11 = 5 have a brother only
• 15 students have a sister and 15 - 11 = 4 have a sister only

So out of 23 students:

• 11 + 5 + 4 = 20 have a brother or sister or both

And therefore

• 23 - 20 = 3

students have no brothers or sisters

The required probability is:

• 3/23