Find the numbers between 1 and 30 that are divisible by 3 or 5:
Divisible by 3:
3, 6, 9, 12, 15, 18, 21, 24, 27, 30
Divisible by 5:
5, 10, 15, 20, 25, 30
There are 10 numbers divisible by 3 and 6 numbers divisible by 5 for a total of 16 numbers.
The probability is 16/30, which reduces to 8/15 are divisible by 3 or 5.
