Question 1110216

The key to solve this is to recall that  Sum(1,2,…, N) =  N*(N+1)/2

Sum(3,6,9,…,60) = 3*Sum(1,2,3,…,20) = 3*(20*21/2) = 630
Sum(1,2,3,…,60) = 61*60/2 = 1830

Sum of natural numbers which are not multiples of 3:  1830-630 = {{{ highlight( 1200 ) }}}