An integer which has two, *and only two*, factors is a **prime number**.

• 0 has no factors, and is not a prime number

• 1 has only 1 factor (1), and is not a prime number

• 2 has two factors (1 and 2) and is a prime number. It is the only even prime number

• 3 is a prime number, as it has factors of 1 and 3

• 4 is not a prime number, as it has three factors: 1, 2 and 4.

The other prime numbers below 20 are: 5, 7, 11, 13, 17 and 19.

To work out whether a number is prime, divide by each of the prime numbers starting with 2. If it is not divisible by any of these numbers, it is a prime number.

What is the next prime number after 20?

21 can be divided by 3 (3 x 7 = 21);

22 can be divided by 2 (2 x 11 = 22),

but 23 can only be divided by 1 and 23.

Answer: 23

Is 49 a prime number?

It is divisible by 1, 7 and 49. As it has three factors, it is not a prime number.

Answer: No

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