Elementary School Lessons

Greatest Common factor or GCF is the largest whole number that divides evenly into each of the numbers.

FACTORS are the numbers that you multiply to get another number. [ 3 X 5 = 15 where the factors are 3 and 5]

To get the greatest common factor, one can use two methods:

• First is to list all the factors of the given numbers. The list the common factors or the same factors for these given numbers. When this has been done, choose the biggest number among the common factors.

ex: 30 = 1, 2, 3, 5, 6, 10, 15, 30

20 = 1, 2, 4, 5, 10, 20

The common factors for 30 and 20 are 1,2, 5 and 10 therefore the greatest common factor is 10.

• Another method for finding the greatest common factor is listing down the prime factors. After listing the prime factors, multiply these prime factors.

PRIME FACTORS are numbers that can only be multiplied by 1 and itself [ex: 2,3,5,7,11…]

The prime factors for 30 are 5 x 3 x 2

The prime factors for 20 are 5 x 2 x 2

The prime factorization of 30 and 20 have one 5 and one 2 in common. Multiply these common prime factors and you get the greatest common factor.

5 x 2 = 10