Prime and composite numbers

The Maths Habit

Is it a prime number? Give a reason.

  1. 41

    Yes: its only factors are 1 and 41

  2. 51

    No: 51 = 3 × 17

  3. 91

    No: 91 = 7 × 13

Lay 12 counters out in equal rows, with more than one row.

Can you do the same with 7 counters?

12: yes, 3 rows of 4 for example. 7: no

Prime
A number with exactly two factors: 1 and itself.
Composite
A number with more than two factors.

Is 13 prime or composite?

List the factors of 18. Is it prime or composite?

factor pairs
1×182×93×6
Factors1, 2, 3, 6, 9, 18
PrimeComposite
18

List the factors of 23. Is it prime or composite?

factor pairs
1×23××
Factors1, 23
PrimeComposite
23

List the factors of 18. Is it prime or composite?

factor pairs
1×182×93×6
Factors1, 2, 3, 6, 9, 18
PrimeComposite
18

List the factors of 27. Is it prime or composite?

factor pairs
1×273×9×
Factors1, 3, 9, 27
PrimeComposite
27

List the factors of 18. Is it prime or composite?

factor pairs
1×182×93×6
Factors1, 2, 3, 6, 9, 18
PrimeComposite
18

List the factors of 21. Is it prime or composite?

factor pairs
1×213×7×
Factors1, 3, 7, 21
PrimeComposite
21

List the factors of 18. Is it prime or composite?

factor pairs
1×182×93×6
Factors1, 2, 3, 6, 9, 18
PrimeComposite
18

List the factors of 31. Is it prime or composite?

factor pairs
1×31××
Factors1, 31
PrimeComposite
31

List the factors of 18. Is it prime or composite?

factor pairs
1×182×93×6
Factors1, 2, 3, 6, 9, 18
PrimeComposite
18

List the factors of 20. Is it prime or composite?

factor pairs
1×202×104×5
Factors1, 2, 4, 5, 10, 20
PrimeComposite
20

Prime and composite numbers worksheet, page 1

Answer sheet

  1. Which is the only even prime number? Why are there no others?

    2

  2. Two prime numbers add to 20. Find both pairs.

    3 + 17, 7 + 13

  3. Max says every whole number is either prime or composite. Give a number that shows he is wrong, and explain why.

    1: it has only one factor, so it is neither

  1. Is 19 prime or composite? Give a reason.

    Prime: factors 1 and 19

  2. Is 49 prime or composite? Give a reason.

    Composite: 7 × 7 = 49

  3. Two prime numbers add to 16. Find both pairs.

    3 + 13, 5 + 11
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