ALGEBRA
posted by Urgen please help .
Choose a natural number between 1 and 30, inclusive. What is the probability that the number is a multiple of 3?

multiples of 3 are
3 6 9 12 15 18 21 24 27 and 30
that is, there are 10 of them
so prob = 10/30 = 1/3
(makes sense, just like the prob that picking a number from 1 to 40, which is a multiple of 4 would be 1/4)