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
