Seven and thirteen also have divisibility rules, but most people don't know them. And there's a reason for that. They aren't particularly practical to use.

In fact, I have to confess that I'd forgotten them, and had to look them up in order to write this article, and in the process discovered that there are several ways of approaching divisibility for 7 and 13...and none of them are any prettier than the one I learned! So, they might be fun to know, but don't expect that you'll use them very much!

Seven: break the number into groups of three digits, starting from the right, and alternately add and subtract the groups. If the result is a multiple of 7, the original number is.

Example: 4,502,715. 715 - 502 + 4 = 217. And since 217 is divisible by 7, so is the original number.

Not very practical, since I used my calculator to do the work, so I might as well have used the calculator from the very beginning to divide the number by 7!

Thirteen: guess what! This one works exactly the same way, except that at the end, you have to check for divisibility by 13 instead of 7.

Example: 1,414,503. 503 - 414 + 1 = 90, which is not divisible by 13, so the original number is not either.