Problem Statement

The objective is to write a function that finds the sum of all positive multiples of 3 or 5 below n. Show output for n = 1000. This is is the same as Project Euler problem 1. Extra credit: do this efficiently for n = 1e20 or higher.

Let's start with the solution:

see sum35(1000) + nl
 
func sum35 n
     n = n - 1
     return(3 * tri(floor(n / 3)) + 
	    5 * tri(floor(n / 5)) - 
	    15 * tri(floor(n / 15)))

func tri n
     return n * (n + 1) / 2