Yo, I don't really have time for that, but what is the summation of the multiples of 3 smaller then 100?

9000

1,683

33

This guy knows his shit

1683

3 + 96 = 99
6 + 93 = 99
9 + 90 = 99

We know that there are 33 such numbers between 0 and 100
so we'll have 16 of these pairs that add to 99
and one odd guy out is 99
so the answer is 99 x 17
Which is (100)(17) - 17 = 1700 - 17 = 1683

This is how you would do it without actually doing it
I realize now that like a billion people answered while I was typing it out though.
It's more practical to use something like this in larger situations like if we were doing between 0 and 1000.

This post was edited by ringo794 on Sep 30 2014 09:56pm

Where did 17 come from

S = n/2 (an + am)

an is the first term
am the last term
n is the total number of terms.

S = 33/2 (3+99)

This post was edited by Whiskers on Sep 30 2014 09:57pm

16 pairs of numbers that add to 99
Then the number 99 itself, which wasn't represented in any of the pairs
that's 17 99's.