Again, thinking back to what these coefficients encode for us, that will be answering our final question that will be counting the total number of subsets whose sum is divisible by five.
So in the 16th Century, Pope Gregory XIII tweak the system again so that leap ears would occur in any year that's divisible by four, but only in centuries that are evenly divided by 400, confusing, I know.
It also can't be 3 above a multiple of 6 unless it's the number 3 itself since all of those are divisible by 3. So at least at the smaller-scale nothing magical is going on.
And all in all the ones that we care about, the subsets with a sum divisible by five, have been put over here on the left and it looks like there's a total of eight of them.
And if we turn to chapter 2 advanced problems, problem number 10 asks this seemingly innocent question " find the number of subsets of the set one up to two thousand, the sum of whose elements is divisible by five" .