CLASS 6 MATHS · PLAYING WITH NUMBERS

Prime numbers, composite numbers and factors

A prime has exactly two factors (1 and itself). A composite number has more than two.

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Worked examples

Example 1 · level 1

Which of these numbers is prime?

28
35
23
40

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28 = 2 × 2 × 7, so it is composite.
35 = 5 × 7, so it is composite.
40 = 2 × 2 × 2 × 5, so it is composite.
23 has no factor other than 1 and 23, so it is prime.

Answer: 23

Example 2 · level 2

Which of these numbers is prime?

99
107
111
129

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129 = 3 × 43, so it is composite.
99 = 3 × 3 × 11, so it is composite.
111 = 3 × 37, so it is composite.
107 has no factor other than 1 and 107, so it is prime.

Answer: 107

Example 3 · level 3

Which of these numbers is prime?

99
133
51
59

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99 = 3 × 3 × 11, so it is composite.
51 = 3 × 17, so it is composite.
133 = 7 × 19, so it is composite.
59 has no factor other than 1 and 59, so it is prime.

Answer: 59