You might remember in your math class a lesson on "divisibility" of numbers. That is, does a given number have other numbers that divide evenly into it. These are sometimes called the "factors" of a number. For example, the number 35 is divisible by 1, 5 and 7. Stated another way, 1, 5, and 7 are "factors" of 35.

As you move along with this concept, there is a list of "rules" you can use to see if a number might have factors. For example, even numbers are all divisible by 2. For 3, you'd add up the digits in a number. If the sum is divisible by 3, then so is the whole number (strange, huh?).

We say "forget the divisility" rules. Testing for divisbility is incredibly easy on the computer using the % or "mod" operator. If you recall from this lesson, the % operator divides two numbers and tells us the remainder (not the quotient). So if use the mod operator on two numbers, say as in A % B, and if we get a zero remainder, then A is divisible by B, or B must be a factor of A!

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