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Least Common Multiple. Relationship of HCF and LCM

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Found in topics: Factorization
Maths Query > Unit > Arithmetic > Number System

Introduction

LCM (Least Common Multiple or Lowest Common Multiple), they all are the same terms. We use them while finding out the number which is the lowest common multiple among many multiples of a number.
To see in more detail how it is calculated, first of all, let’s review some general terms related to LCM.

Multiple of a number

A multiple is a number that we can get from a number when multiplied by any positive integer.

Example

What are the multiples of 7?
We can say there are unlimited multiples of 7.

To be short, 14 is a multiple of 7.
Why?
Because, here, 7 is the number
when 7 is multiplied by 2, the obtained number 14 is multiple 7.
How?
7 × 2 = 14
Still, we can create more multiples of 7 by multiplying 7 with positive integers such as 3, 4, 5, 6, 7, 8….. and so on
7 × 3 = 21
7 × 4 = 28
7 × 5 = 35
7 × 6 = 42
7 × 7 = 49
7 × 8 = 56
So, here, 21, 28, 35, 42, 49, 56 are also multiples of 7.

Common multiples of numbers

Common multiples are calculated for two or more than two numbers. They are the multiples which are common among the multiples of the given numbers.

Example

Let’s understand it by taking the two numbers as 2 and 4 to find their common multiples.
Step1: find out the multiples of 2 and 4 separately.

Multiples of 2:
2 × 1 = 2
2 × 2 = 4
2 × 3 = 6
2 × 4 = 8
2 × 5 = 10
2 × 6 = 12
The multiples of 2 are 2, 4, 6, 8, 10, 12 and so on.
Multiples of 4:
4 × 1 = 4
4 × 2 = 8
4 × 3 = 12
4 × 4 = 16
4 × 5 = 20
4 × 6 = 24
The multiples of 4 are 4, 8, 12, 16, 20, 24 and so on.
Step2: Find out the common multiples of 2 and 4.
Finally, we can say 4, 8, 12 are the common multiples of 2 and 4, because the multiples 4, 8 and 12 do exist for both numbers 2 and 4.

How to find LCM?

LCM (Least Common Multiple or Lowest Common Multiple) is the smallest number among the common multiples of given numbers.

Example

To understand LCM, lets reconsider the above example of common multiples, but now finding here the LCM of 2 and 4.
Find the LCM of 2 and 4.
So, what are the steps?
Step 1: Find out the multiples of 2.
Step 2: Find out the multiples of 4.
Step 3: Find out the common multiples of 2 and 4.
Step 4: Find out the smallest number among those common multiples, that will be the LCM of 2 and 4.
Step 1: Find out the multiples of 2.
2 × 1 = 2
2 × 2 = 4
2 × 3 = 6
2 × 4 = 8
2 × 5 = 10
2 × 6 = 12
The multiples of 2 are 2, 4, 6, 8, 10, 12 and so on.
Step 2: Find out the multiples of 4.
4 × 1 = 4
4 × 2 = 8
4 × 3 = 12
4 × 4 = 16
4 × 5 = 20
4 × 6 = 24
The multiples of 4 are 4, 8, 12, 16, 20, 24 and so on.
Step 3: Find out the common multiples of 2 and 4.
Therefore, the common multiples are 4, 8 and 12.
Step4: Find out the smallest number among those common multiples, that would be the LCM of 2 and 4.
Therefore, the smallest number among common multiples 4, 8 and 12 is 4.
Hence, LCM of 2 and 4 is 4.

Relationship between LCM and HCF

Product of LCM and HCF

Product of LCM and HCF of given numbers is equal to the product of the numbers.

Formula

LCM × HCF of two numbers = Product of the two numbers

Frequently Asked Questions

1) What is LCM?

LCM stands for Least Common Multiple or Lowest Common Multiple. It is the smallest number that is obtained from the common multiples of given number.

2) What are the methods to find LCM?

1) Prime factor method
2) Common division method
3) Common multiple method

3) What is the relationship between HCF and LCM?

For any numbers a and b: Product of their LCM and HCF = Product of the numbers

Solved Examples

1) Find LCM of 12 and 24.

We can find LCM by common multiple method
Multiples of 12 are 12, 24, 36, 48, 60.
Multiples of 24 are 24, 48, 72, 96, 120.
From the above, the common multiples are 24, 48 and the lowest number of multiple is 24.
Therefore, LCM of 12 and 24 is 24.

2) Find LCM of 18 and 20 using prime factorization method.

Prime factorization of 18 = 2 × 3 × 3 = 2 × 32
Prime factorization of 20 = 2 × 2 × 5 = 22 × 51
From the above, common factors = 2.
To find the LCM, we take the highest power of common factor i.e. 22
∴ LCM of 18 and 20 = 22 × 32 × 51
= 4 × 9 × 5
= 180

3) Find LCM of 18 and 20 using common division method.

Let's find LCM using common division method. LCM division method
LCM = 2 × 2 × 3 × 3 × 5
LCM = 4 × 9 × 5
= 180

4) Find LCM of 18 and 20 using common multiple method.

Multiples of 18 = 18, 36, 54, 72, 90, 108, 126, 144, 162, 180
Multiples of 20 = 20, 40, 60, 80, 100, 120, 140, 160, 180, 200
Common multiple = 180
∴ LCM of 18 and 20 = 180

5) If two numbers are in the ratio 9:10 and their HCF is 2. Find their LCM.

Let the numbers are 9x and 10x.
where HCF = x = 2
∴ LCM = 9 × 10 × x
LCM = 90x
or LCM = 90 × 2 = 180

6) Three numbers are in the ratio 2:3:4 and their LCM is 120. Find their HCF.

Let the numbers are 2x, 3x and 4x.
where HCF = x
∴ LCM = 22 × 3 × x
LCM = 4 × 3 × x = 12x
But, LCM = 120 (..............given)
or 12x = 120
or = 120 12
x = 10

Worksheet 1

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Fill in the blanks

1) The fourth multiple of 15 is ___________.

2) The ___________ of two prime numbers is equal to their product.

3) The product of ___________ and LCM is equal to the product of their given numbers.

4) The LCM of 7 and 13 is ___________.

5) The LCM of 4 and 12 is ___________.

6) The eighth multiple of 18 is ___________.

7) HCF of 5 and 8 is 1 and the product of 5 and 8 is 40, then the value of LCM is ___________.

8) The LCM of two coprime numbers is equal to their ___________.

9) If two numbers are in the ratio 2:3 and their HCF is 5, then the LCM of two numbers will be ___________.

10) Every number is ___________ of itself.

Help iconHelp box
product
91
40
LCM
HCF
30
60
12
144
multiple

Worksheet 2

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Solve the questions

1) Write the first five multiples of 9.

2) Write the first seven multiples of 6.

3) Write the first four multiples of 10.

4) Write the first four common multiples of 9 and 18.

5) Find LCM of 12, 20 and 14.

6) If three numbers are in the ratio 1:2:3 and its HCF is 13, find its LCM.

Worksheet 3

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Multiple choice questions

1) Common multiples of 6 and 12 are

  1. 12, 24, 36
  2. 12, 18, 30
  3. 6, 12, 18
  4. 30, 36, 48

2) LCM of two coprime numbers x and y is

  1. xy
  2. xy
  3. x + y
  4. x - y

3) LCM of two prime numbers a and b is

  1. ab
  2. ab
  3. a + b
  4. a - b

4) LCM of 3 and 2 is

  1. 6
  2. 3
  3. 1
  4. 2

5) The HCF of two numbers is 2. The LCM of two numbers is 12. If one of the number is 6 then the other number will be

  1. 6
  2. 2
  3. 12
  4. 4

6) The LCM of 4 and 5 is

  1. 4
  2. 5
  3. 20
  4. 1

7) The LCM of 3, 4 and 5 is

  1. 60
  2. 3
  3. 4
  4. 5

8) If the product of two numbers is 50. HCF of two numbers is 5, the LCM equals to

  1. 10
  2. 5
  3. 50
  4. 1

9) HCF of given numbers must divide their LCM

  1. True
  2. False
  3. Both
  4. None of these

10) The smallest number which is completely divisible by 24 and 40 is

  1. 60
  2. 40
  3. 24
  4. 120
MCQ Answer Key Hide Show
1. a
2. a
3. a
4. a
5. d
6. c
7. a
8. a
9. a
10. d
Last updated on: 30-07-2024

Related chapters

Highest Common Factor or GCF. Properties of HCF
Fundamental Theorem of Arithmetic with Applications
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