Least Common Multiples (LCMs)

A common multiple of two whole numbers a and b is a number c which a and b both divide into evenly.

For example, 48 is a common multiple of 6 and 12 since

48÷6=8   and

48÷12=4 .

The least common multiple is just what it sounds like... the smallest of all the common multiples.

Example 1:

Find the least common multiple of 9 and 12 .

To do this, we can list the multiples:

9:9,18,27, 36 _ ,45,54,63,72,... 12:12,24, 36 _ ,48,60,72,...

36 is the first number that occurs in both lists. So 36 is the LCM.

The listing method is impractical for large numbers. Another way to find the LCM of two numbers is to divide their product by their greatest common factor (GCF).

Example 2:

Find the least common multiple of 18 and 20 .

To find the GCF of 18 and 30 , you can write their prime factorizations:

18=233 30=235

The common factors are 2 and 3 . So, the GCF is 23=6 .

Now find the LCM by multiplying the two numbers and dividing by the GCF. (You can make this calculation a little easier by cancelling a common factor.)

1830 6 = 3630 6 =90

When the GCF of two numbers is 1 , the LCM is equal to the product of the two numbers.

Example 3:

Find the least common multiple of 10 and 27 .

10 and 27 share no common factors other than 1 . So, the GCF is 1.

Therefore, the LCM is simply 1027=270 .

A third way to find the LCM of is to list all of the prime factors of each number and then multiply all of the factors the greatest number of times each occurs in any of the lists. [Note that while the previous method won't always work with more than 2 numbers, this method will.]

Example 4:

Find the LCM of 16,25 and 60 .

16=2222 25=55 60=2235

The greatest number of times the factor 2 occurs is four (in the first list).

The greatest number of times the factor 3 occurs is one (in the third list).

The greatest number of times the factor 5 occurs is two (in the second list).

So, we multiply four 2 s, one 3 , and two 5 s.

LCM =2222355=1200