If 3, 6, 9, 12, 15, and 18 are multiples of 3 between 1 and 20, and 5, 10, 15 and 20 are multiples of 5 between 1 and 20, then you add the amount of multiples together to get 10.
10 over 20 as a fraction can be simplified to 1 over 2 which is 50%.
This means that there is a 50% chance of drawing out a multiple of 3 or 5.
