Pre-Algebra Demystified

Another method is to list the multiples of the number in the denominator

and eventually you will find a common multiple which is the same number as

a common denominator.

Amultipleof a given number is the product of the given number and any

other whole number. Multiples of a given number are obtained by multi-

plying the given number by 0, 1, 2, 3, 4, 5, etc. For example, the multiples of 5

are

5  0 ¼ 0

5  1 ¼ 5

5  2 ¼ 10

5  3 ¼ 15

5  4 ¼ 20

5  5 ¼ 25

etc.

The multiples of 6 are

6  0 ¼ 0

6  1 ¼ 6

6  2 ¼ 12

6  3 ¼ 18

6  4 ¼ 24

6  5 ¼ 30

etc:

Now, if you want to find a common denominator, simply list the multiples

of the numbers in the denominators until a common multiple of both num-

bers is found. For example, the common denominator of^15 and^16 is found as

follows:

0 ; 5 ; 10 ; 15 ; 20 ; 25 ; 30 ;

0 ; 6 ; 12 ; 18 ; 24 ; 30 ;

Since 30 is the smallest common multiple of 5 and 6, it is the lowest

common denominator of^15 and^16.

When the denominators of the fractions are large, another method, called

thedivision method, can be used. The procedure for finding the lowest com-

mon denominator of two or more fractions is as follows:

54 CHAPTER 4 Fractions: Part 2