This paper studies the numeral divisibility characteristic of round number, and solves the problem in theory about the divisibility characteristic of any round number.
I guess there are more of them that work nicely in base twelve because there are more numbers that divide exactly into twelve. - Two, three, four, six.
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.
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.
Eratosthenes of Cyrene was a 3rd century BCE mathematician from ancient Greece interested in prime numbers, that is numbers only divisible by 1 and themselves.
Cyrene 埃拉托色尼 (Eratosthenes of Cyrene) 是公元前 3 世纪来自古希腊数学家,他对素数很感兴趣,素数只 1 和它们本身整除。