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Find the LCM and the GCF of Integers - Examples and Questions with Answers (Grade 5)

How to find the LCM and GCF on integers? Examples and questions with answers.

This is a tutorial on how to find the LCM and GCF. In order to help you check your answers, a calculator for the LCM and a calculator for the GFC of two numbers are provided.

Lowest Common Multiple (lcm) Calculator. Calculate the lowest common multiple of two positive integers.

Greatest Common Factor (gcf) Calculator. Calculate the greatest common factor of two positive integers.

A - Find the LCM of two Numbers



Method 1: Find the LCM of 6 and 4 listing the multiples.

Find the first few multiples of the two numbers as follows:

6: 6 , 12 , 18 , 24 , 30 ,...

4: 4 , 8 , 12 , 32 , 40 ,...

The lowest multiple that is common to 6 and 4 is 12. So the LCM of 6 and 4 is 12.

The above method works well for small numbers only.

Method 2: Find the LCM using prime factorization.

Example 1: Find the LCM of 6 and 4 using prime factorization.

Find the prime factorization of the two numbers.

6 = 2 x 3

4 = 2 x 2

A common multiple may be found by multiplying the two numbers 6 and 4. However it may not be the lowest. Let us multiply the two numbers in factored form : (2 x 3) x (2 x 2). One factor 2 is counted twice and therefore has to be taken out of one term of the product if we want our common multiple to be the lowest. The LCM of 4 and 6 = 3 x (2 x 2) = 12.

Example 2: Find the LCM of 20 and 24 using prime factorization.

Find the prime factorization of the two numbers.

20 = 2 x 2 x 5

24 = 2 x 2 x 2 x 3

The product of 20 and 24 is given by (2 x 2 x 5) x (2 x 2 x 2 x 3)

2 x 2 is counted twice and therefore has to be taken out of one term and so the LCM is given by

LCM = (5) x (2 x 2 x 2 x 3) = 120

Example 3: Find the LCM of 1240 and 5300 using prime factorization.

Find the prime factorization of the two numbers.

1240 = 2 x 2 x 2 x 5 x 31

5300 = 2 x 2 x 5 x 5 x 53

The product of 1240 and 5300 is given by (2 x 2 x 2 x 5 x 31) x (2 x 2 x 5 x 5 x 53)

2 x 2 x 5 is counted twice and therefore has to be taken out of one term and so

LCM of 1240 and 5300 = ( 2 x 31) x (2 x 2 x 5 x 5 x 53) = 328,600


A - Find the GCF of two Numbers



Example 1: Find the GCF of 6 and 4 using prime factorization.

6 = 2 x 3

4 = 2 x 2

2 is a common factor to both 6 and 4 and it is the highest. So the GCF of 6 and 4 is equal to 2.

Example 2: Find the GCF of 20 and 24 using prime factorization.

20 = 2 x 2 x 5

24 = 2 x 2 x 2 x 3

Underline all common factors in the factorization of 20 and 24.

20 = 2 x 2 x 5
24 = 2 x 2 x 2 x 3

The greatest common factor of 20 and 24 is the product of all common factors 2 x 2 = 4.

Example 3: Find the GCF of 1240 and 5300 using prime factorization.

1240 = 2 x 2 x 2 x 5 x 31

5300 = 2 x 2 x 5 x 5 x 53

Underline common factors

1240 = 2 x 2 x 2 x 5 x 31

5300 = 2 x 2 x 5 x 5 x 53

The GCF of 1240 and 5300 is given by 2 x 2 x 5 = 20


C - Relationship Between The LCM and the GCF of two numbers



The LCM's and GCF's found above are organized in the table. It is shown that the product of any two whole numbers is equal to the product of their LCM and GCF.

(m , n) LCM(m,n) GCF(m,n) m x n LCM(m,n) x GCF(m,n)
(4 , 6) 12 2 24 24
(20 , 24) 120 4 480 480
(1240 , 5300) 328,600 20 6,572,000 6,572,000


m x n = LCM(m,n) x GCF(m,n)



Exercise: Find the LCM and GCF of each pair of numbers and check that the product of the two numbers is equal to the product of the LCM and the GCF.

  1. 21 , 14
  2. 45 , 55
  3. 120 , 248

answers

Answers to the Above Questions

  1. GCF = 7 , LCM = 42
  2. GCF = 5 , LCM = 495
  3. GCF = 8 , LCM = 3720
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Updated: 14 March 2009 (A Dendane)