Question

Find all integer numbers n, such that 1+2+3+...+n is divisible by 5

Answer 1

Any number that ends in 0 like, 10, 20, 30, ect. and numbers that end in 5 like, 25, 35, 45, are all divisible by 5.

Explanation:

So if any of the sums end in 0 or 5 it is divisible by 5.