Explore · Mathematics · Multiplication & Division

Prime numbers

Know and use the vocabulary of prime numbers, prime factors, and composite numbers; establish whether a number up to 100 is prime; recall prime numbers up to 19

ages 9 to 108 practice questionsvideo included

What a learner can do afterwards

Explain that a prime number has exactly two factors: 1 and itself
Determine whether 51 is prime or composite and justify the answer
List all prime numbers up to 19 from memory

The lesson

You already know a factor is a number that divides exactly into another number, with nothing left over. A composite number has three or more factors. A prime number is stricter: it has exactly two factors, 1 and itself. That is why 1 is not prime, it only has one factor, itself.

Tap to fill the grid, one at a time: 2 rows of 3.

Six dots make a neat rectangle, 2 rows of 3. That extra factor makes 6 composite. A prime number like 7 can only ever line up in a single row.

Try it together

Is 51 prime or composite? Try dividing by small primes. 51 divided by 3 is 17 exactly, with nothing left over. So 51 has the factors 1, 3, 17, and 51, four factors in total. That is more than two, so 51 is composite.

These eight numbers are the only primes up to 19. Try saying them from memory: 2, 3, 5, 7, 11, 13, 17, 19.

Good to know

To test any number up to 100, you only need to try dividing it by 2, 3, 5, and 7. If none of those divide it exactly, the number is prime.

A prime number has exactly two factors, 1 and itself, and you can test any number up to 100 by checking a few small primes.

Watch it

Where it sits

Learn first

Factors, multiples, and primes

This opens up

Nothing builds on it yet.

Then practise

8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.