# Multiples and factors

When “a” divided by “b” is exact, we say that **“a” is a multiple of “b”**, or **“b” is a factor (or divisor) of “a”**, or** “a” is divisible by “b”**.

Example: 3 is a factor (or divisor) of 15 and 15 is a multiple of 3.

We write: Div (15) or

We calculate the multiples of a number by multiplying this number by another number. Example:

A number is **prime** if it has exactly two factors: 1 and itself. If the number is not prime, it is **composite**.

Example: 20 is composite and 13 is prime.

**Exercises**:

1.- Calculate the first five multiples of 12 and 13

2.- Calculate all the factors of 28 and 42

3.- Decide if the following numbers are prime or composite numbers: 71, 221 and 233

Solutions:

1.- 12→{12,24,36,48,60} 13→{13,26,39,52,65}

2.- fact(28) = {1,2,4,7,14,28} fact(42) = {1,2,3,6,7,14,21,42}

3.- 71 and 233 are prime numbers and 221 is composite because 221 = 13·17

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