Question

In a class of
25
25 students,
3
5
5
3
​
of the class are boys,
2
5
5
2
​
of the class have ​blonde hair, and
1
5
5
1
​
of the class are boys with blonde hair.

​

​If one student is ​to be chosen at random from the class, what is the probability that the ​student is a boy or has blonde hair?

​Show your work on the scratchpad.

​

​P (boy or blonde hair) =

Answer 1

Scratchpad:
- Total students = 25
- Boys = (3/5)·25 = 15
- Blonde hair = (2/5)·25 = 10
- Boys with blonde hair = (1/5)·25 = 5

Use inclusion–exclusion:
P(boy or blonde) = P(boy) + P(blonde) − P(boy and blonde)
= 15/25 + 10/25 − 5/25 = 20/25 = 4/5

P(boy or blonde hair) = 4/5 (80%).