A composite number is any number that has factors besides itself and 1. In other words, a composite number is a number that you can divide to get a whole number, as long as you don't use the same number or the number 1 to divide it. It's easier to explain with the example above.
Let's work backwards, and start with 21. Can you divide 21 by 1? Obviously, yes, to get 21; but that doesn't count when you're trying to find a composite number. What about 2? That doesn't go in evenly. 3? 21/3=7, which is a whole number. Since it can be divided by a whole number other than 1 or 21, it's composite. See where I'm getting at?
What about 15? 15 divided by 2 isn't a whole number. But 15 divided by 3 is 5, so 15 is composite.
9 can't be evenly divided by 2. But 9 divided by 3 is 3! Another composite number.
What about 2? It can be divided by 1 and 2. But wait a second: 2 is itself! We can only divide 2 into 1 and 2. A number is only composite if it can be divided by something other than 1 or itself, which in this case is 2. So, 2 is NOT a composite number. We actually call it a PRIME number, because it's not composite!
Answer: 2
