Give answer as a fraction in simplest form

Question

A bag contains 40 slips of paper numbered 1-40. If one is randomly chosen, what is the probability of not choosing a multiple of 3?

Answer 1

P(not 3n) = 1-Pr(3n)

3n includes 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, and 39. 40/3 = 13 1/3

13 ways out of 40

P(not 3n) = 1-13/40 = 27/40.