1
### Meaning

2, 7 and 14 are common factors of 28, 42 and 70; 14 is the greatest of the three common factors. We say that 14 is the highest common factor of 28, 42 and 70. HCF is short for Highest Common Factor.

2
### Procedure

To find the HCF of a set of numbers:
a. express the numbers as a product of prime factors;
b. find the common prime factors;
c. multiply the common prime factors together to give the HCF.

3
### Example

Find the HCF of 18, 24 and 42.
18 = 2 x 3 x 3
24 = 2 x 2 x 2 x 3
42 = 2 x 3 x 7
The common prime factors are 2 and 3.
The HCF = 2 x 3 = 6.

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