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example 2. Find the common factors of 12 and 20.

Solution. 12 can be written as: 20 can be written as:
12 = 1 × 12 20 = 1 × 20

12 = 2 × 6 20 = 2 × 10
12 = 3 × 4 20 = 4 × 5

Factors Factors 6 1 5
3 2 4 10
The factors of 12 are 1, 2, 3, 4, 6 and 12.
12 20
The factors of 20 are 1, 2, 4, 5, 10 and 20. Factors Factors

So, the common factors of 12 and 20 are 1, 2 and 4. of 12 Common factors of 20

Exercise 5D

1. Find the factors of the following numbers first and then write their common factors:

a. 6 and 10 Common factors
Factors of 6 = ___________

Factors of 10 = ____________
Factors Factors
Common factors = ___________ of 6 of 10

b. 18 and 21 Common factors

Factors of 18 = ___________
Factors of 21 = ___________ Factors Factors

Common factors = ___________ of 18 of 21

c. 10 and 20 Common factors
Factors of 10 = ___________

Factors of 20 = ___________ Factors Factors
Common factors = ___________ of 10 of 20

d. 18 and 32 Common factors

Factors of 18 = ___________
Factors of 32 = ___________ Factors Factors

Common factors = ___________ of 18 of 32

2. Find the common factors of the following numbers.
a. 21 and 36 b. 18 and 54 c. 34 and 35 d. 15 and 25

e. 14 and 35 f. 56 and 64 g. 16 and 20 h. 18 and 40

88 Mathematics 4