Question 1076650
3*1 = 3
3*2 = 6
…
3*33 = 99

So there are 33 multiples of 3 less than 100.   To find the sum, use {{{ sum(i, i=1,i=n) = (n*(n+1))/2 }}} and then multiply the result by 3:

 3+6+9 +… + 99  =  3(1+2+3+…+33) = {{{ 3*sum(i, i=1, i=33) = 3*(33*34/2) = highlight(1683) }}}
  
To find the sum of numbers less than 100 that are not multiples of 3, I would take the approach of finding the sum of all the numbers less than 100 {{{ sum(i, i=1, i=99) = 99*100/2 }}}  and then subtract from that result, the sum of the multiples of 3 we found above.