Question

Find the sum of all the numbers between 50 and 500 which are divisible by 3 and 5

Answer 1

Hello,

The sum is 8325.

Explanation:

Those numbers are 60,75,90,...,495.

There is (495-60)/15+1=30 numbers

Like Gauss, we add twice those numbers :

(60+495)*30/2= 8325.