Remember when you learned about factors?  Review this lesson if you are unsure of the definition of factors, because you need to know what they are!

Every whole number greater than 1 is either a prime number or a composite number, depending upon how many factors the number has.  Now you need to know which numbers are which! 

An example of a prime number is 7.  It can be divided evenly by only 1 and itself. (When divided by any other number, it leaves a remainder).  To find all prime numbers between 1 and 100, follow the Practice sheet using the 100 chart  link below.   Other examples of Prime Numbers are 2,3,5,7,11,13,17, 19, and 23.

An example of a composite number is 4,  because it has 1,2 and 4 as factors. ( 4÷2 = 2 with no remainder).  Other examples are 6,8,9,10, 12, 14, 15, 16 and many more.  Each of those numbers is divisible by 1, itself, and some other number(s).

Determine if the number is even, because even numbers  automatically have 2 as a factor, plus themselves and 1, and are therefore composite.  So, all even numbers greater than 2 are composite.

But then can you automatically say that all odd numbers are prime? NO,  many odd numbers have other factors.  e.g.) 9 ÷ 3 = 3, therefore 9  has 1,3,and 9 as factors and is composite.  What about 15?  It is odd, but can you think of factors of 15 other than 1 and 15?   Since 15 ÷ 3 = 5,  15 has 1,3,5,and 15 as factors and is therefore composite, not prime.

Try some more!  State if the following numbers are prime or composite: