Alicia road these numbers to 2, 9, 15, 21.
which of alicia's numbers is not a composite number?

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!

2