JAMB 1985 · UME · Q35

What is the probability that a number chosen at random from the integers 1 to 10 inclusive is either a prime or a multiple of 3?

Worked solution (try it first)

The primes from 1 to 10 are 2, 3, 5, 7, and the multiples of 3 are 3, 6, 9.
Combine them, writing 3 only once: $\{2, 3, 5, 6, 7, 9\}$ , which has 6 members.
So the probability is $\frac{6}{10} = \frac35$ , option B.