Question

if one of the numbers 1 to 20 is chosen at random what is the probability that it is either a multiple of 3 or a multiple of 5 or both?

Answer 1

45% that either would occur, or a 5% chance both would occur.

Explanation:

There is one overlap, 15, so it must be subtracted from one of the number lists.

3 6 9 12 15 18

5 10 15 20

6/20 + 3/20 = 9/20 = 0.45 = 45%

15 is the ONLY overlap number, so 1/20 times both would occur.

1/20 = 0.05 = 5%