

Least Common Multiple
The Least Common Multiple (L.C.M.) of two numbers is the lowest number that can be divided
both. It can be used to find the lowest common denominator when adding or subtracting
fractions. Don't let the "least" in the name fool you  the LCM is no smaller than the largest of the numbers.
HINTS:


LEAST  lowest or smallest.
COMMON  something shared or in common.
MULTIPLE  what you get when you multiply.

EXAMPLE:
Find the Least Common Multiple (L.C.M.) of 4 and 6. 

Multiples of 4:
 4,
 8,
 12,
 16,
 20,
 24,
 28, 32....


1*4, 
2*4, 
3*4, 
4*4, 
5*4, 
6*4, 
and so on

Multiples of 6:
 6,
 12,
 18,
 24,
 30,
 36,
 42, 48....

 1*6, 
2*6, 
3*6, 
4*6, 
5*6, 
6*6, 
and so on 

Since 12, 24, 36, 48.... can be all divided by BOTH 4 and 6, each of them is a common multiple (and could be used as a common denominator of both fourths and sixths, for example). The lowest
common multiple is 12, therefore it is the Least Common Multiple (L.C.M.) of 4 and 6.

You can also use the prime factorization method to find the Least Common Multiple:
EXAMPLE:
Find the Least Common Multiple (L.C.M.) of 10 and 12:


Count the maximum number of times each factor appears in either quantity. The product of those factors is the
Least Common Multiple (L.C.M.):

Usually you can find the Least Common Multiple fairly easily by experimentation. Start with
the larger number: Will the other number divide into it? If so, you have the L.C.M.; if not:
What is the next multiple of the larger number  is IT divisible by the other number? Continue
until you find a number that is divisible by both numbers. If they share no common factors (other than one) then the
Lowest Common Multiple will be the product of the two numbers.

For example the L.C.M. of 5 and 8 is 40
since the only common factor is one,
just multiply the numbers: 5*8 = 40.

Find the Lowest Common Multiple (L.C.M.) of 6 and 12.
Will 6 go into 12? Yes.... L.C.M.= 12

Find the Lowest Common Multiple (L.C.M.) of 6 and 10.
Will 6 go into 10? No.... Next Multiple 2*10 = 20
Will 6 go into 20? No.... Next Multiple 3*10 = 30
Will 6 go into 30? Yes.... L.C.M. = 30

PRACTICE: ("Click" on Check answer or use the answer key below.)

ANSWERS:
1) C
2) D
3) A
4) C
5) B
6) B
7) E
8) A

© 1999 Amby DuncanCarr All rights reserved.
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http://amby.com/educate/math/21_LCM.html
