If we list all the natural numbers up to but not including 20 that are multiples of either 3 or 5, we get 3, 5, 6 and 9, 10, 12, 15, and 18.

The sum of these multiples is 78.

Write a program that can find the sum of the multiples of a given set of numbers. If no set of numbers is given, default to 3 and 5.