The sum of all positive integers less than 400 which are not divisible by 5.

We will use the sum of natural number series formula 
if S=1+2+3+4++n, then
 S= n(n+1)/2
 Now, sum of all +ve integers from 0 to 400 is given by
 S=0+1+2+3++400
 => S=400(400+1)/2;
 =>S=80200;
 Sum of all +ve integers divisible by 5 is given by
 R=0+5+10+15+20+25+30++400
 R=5(1+2+3+4+5+6++80)
 R=5*80(80+1)/2
 R=16200 ;
 therefore the sum of all +ve integers <400 and not divisible by 5=S-R=64000