FactorsPrime Factors

Multiples

Common Factors and Highest Common Factor (HCF)

Factors and Multiples

Common Multiples and Lowest Common Multiple (LCM)

1. FactorsFactors of a given whole number are the numbers that can divide the given whole number exactly.

e.g. Factors of 12 = 1, 2, 3, 4, 6, 12 Factors of 19 = 1, 19

List all the factors of each of the following numbers.

4312621212112

The factors of 12 are 1, 2, 3, 4, 6 and 12 .

Example

1 12

74281422828128

The factors of 28 are 1, 2, 4, 7, 14 and 28 .

2. 28

3. 1717117

The factors of 17 are 1 and 17 .- The number 1 is a factor of all

the whole numbers.- Every number is a factor of itself.

Example 2

Determine whether 9 is a factor of each of the following numbers.(a) 63 (b) 1963solution(a) 63 9 = 7

Therefore, 9 is a factor of 63.

An exact division

(b) 1963 9 = 218 remainder 1

Not an exact division

Therefore, 9 is not a factor of 1963.

Try Question in exercise

1. List all the factors of each of the following numbers.(a) 10 (b) 13 (c) 36

(d) 52(e) 75 (f) 84 (g) 100

(h) 1252. Determine whether each of the

following numbers is a factor of the number in brackets.(a) 6 (54) (b) 8 (68)(c) 12 (102) (d) 13 (156)

Exercise

2. Prime FactorsPrime factors of a given whole

number are the factors of the given whole number which are also prim numbers.

e.g. Prime factors of 12 = 2, 3

Find all the prime factors of 20.e.g. 2 )20

2 )10 5 ) 5 1

Therefore : The prime factors of 20 are 2 and 5 .

Keep dividing by the smallest prime factor until you get 1.

Example 1

Hot tipsThe factor tree method can also be used to find the prime factors of a given whole number.For example.

202 10

2 x 2 x 5

From the factor tree method, we have 20 = 2 x 2 x 5.Therefore, the prime factors of 20 are 2 and 5.

1. Find all the prime factors of each of the following numbers.(a) 12 (b) 15 (c) 28(d) 42 (e) 231 (f) 330

2. Express each of the following numbers as a product of its prime factors.(a) 18 (b) 30 (c) 57(d) 78 (e) 255 (f) 441

Exercise

3. Common Factors and Highest Common Factor (HCF)

A common factor is number that is a factor of two or more numbers.

e.g. Common factors of 6 and 18 = 1, 2, 3, 6

Example 1

18 and 32

Example 2

The common factors of 4, 8 and 16 are 1, 2 and 4

Factors of 32 :The common factors of 18 and 32 are 1 and 2

1, 2

, 3

, 6

, 9

, 18,

2

1, 4

, 8

, 16

, 32

Factors of 18 :

4, 8 and 16Factors of 8:

Factors of 4 :Factors of 16 :

1 , 2

, 4

1 , 2

, 4

1 , 2

, 4

, 8, 8

, 16

Hot tipsThe number 1 is a common factor of all whole numbers.

Exercise

1. Find all the common factors of each of the following sets of numbers.(a) 21 and 36 (b) 42 and 63(c) 8, 12 and 16 (d) 30, 60 and 800

A highest common factor (HCF) is the greatest common factor of two or more numbers.

Example 1

HCF of 4, 6 and 16 are 4

The common factors of 4, 8 and 16 are 1, 2 and 4

Find the highest common factor of 4, 8 and 16Factors of 8:

Factors of 4 :Factors of 16 :

1 , 2

, 41 ,

2, 41 ,

2, 4

, 8, 8

, 16

Method 1: List all factors

solutio

n

Exercise

Find the HCF of each of the following sets of numbers

by listing all the factors.(a) 18 and 24 (b) 32 and 80(c) 6, 16 and 24 (d) 35, 56 and 84

Example 2

x 3

Find the highest common factor of 12 and 1812

=18 =

The HCF of 12 and 18 is 2 x 3 = 6

Method 2: Prime factorisation

2 x 2 x 32 x 3 x 3

Identify the prime factors which are common factors of both numbers.

solution

2

Exercise

Find the HCF of each of the following sets of

numbers using prime factorisation. (a) 42 and 54 (b) 56 and

98(c) 6, 24 and 84 (d) 30, 55 and 70

1

Example 3

Method 3: Algorithm

Find the highest common factor of 18, 27 and 36 3 ) 18 27

36 3 ) 6 9 122 3

4

Stop dividing when there are no more common factors except 1.

The HCF of 18, 27 and 36 is 3 x 3 = 9 Multiply all the

divisors.

solution

Exercise

Find the HCF of each of the following sets of

numbers using algorithm.(a) 44 and 66 (b) 45 and 90(c) 9, 21 and 33 (d) 48, 64 and 80

4. MultiplesA multiple of a given whole number is the product of itself and another non – zero whole number.

For example, multiples of 3:

3 x 1 = 33 x 2 = 63 x 3 = 9: : :

Multiples of 3 = 3, 6, 9, 12, 15, …

5. Common Multiples and Lowest Common Multiple (LCM)

A common multiple is a multiple of two or more whole numbers.

e.g. Common multiples of 2 and 3 = 6, 12, 18, …

Exercise

List the first three common multiples of each of the

following sets of numbers.(a) 3 and 4 (b) 2 and 5(c) 2, 3 a$nd 4 (d) 4, 6 and 8

A lowest common multiple (LCM) of two or more numbers is the smallest common multiple of these numbers.

Example 1

Method 1: List all multiples

Find the lowest common multiple of 4 and 6Multiples of 4 :Multiples of 6 :

4, 8,12, 16, 20, 24, …6, 12, 18, 24, …The LCM of 4 and 6 is

12.

solution

Exercise

Find the LCM of each of the following sets of

numbers by listing all the multiples.(a) 4 and 5 (b) 8 and 12(c) 2, 4 and 9 (d) 18, 24 and 48

Example 2

Method 2: Prime factorisation

Find the lowest common multiple of 4 and 6

4 = 2 x 26 = 2 x 3

x 3The LCM of 4 and 6 is 2 x 2

x 3 = 12

Ensure that all the factors are prime numbers.

solution

2x 2

Exercise

Find the LCM of each of the following sets of

numbers using prime factorisation. (a) 7 and 9 (b) 25 and 30

(c) 10, 20 and 35 (d) 18, 24 and 32

2 ) 6 8 30 3 ) 3 4 15 4 ) 1 4 5 5 ) 1 1 5 1 1 1

LCM of 6, 8 and 30 = 2 x 3 x 4 x 5 = 120

Keep dividing by the smallest prime factor until you get 1. Carry down the numbers that are not divisibled by the prime numbers.

Multiply all the divisors.

Example 3

Method 3: Algorithm

Find the lowest common multiple of 6, 8 and 30

solution

Exercise

Find the LCM of each of the following sets of numbers using algorithm.

(a) 4 and 14 (b) 36 and 82(c) 6, 16 and 64 (d) 21, 28 and 70