 What you need to know

Things to remember:

• You need to know the prime numbers between 1 and 20
• 1 isn’t a prime number
• 2 is the only even prime number

A prime number is a positive whole number bigger than 1 that only has two factors, itself and 1.

For example, 13 is a prime number because its only factors are 1 and 13.

You need to learn the prime numbers between 1 and 20:

2                      3                      5                      7

11                    13                    17                    19

2 is a special prime, because it is the only even prime. All other even numbers can be divided by 2, so can’t be prime.

You need to be able to tell if a number smaller than 100 is prime.

Is 59 a prime number?

If you can’t divide a number less than 100 by 2, 3,5, or 7 and get a whole number, then it is prime.

$$59\div2=29.5$$

$$59 \div 3 = 19.6666...$$

$$59 \div 5 = 11.8$$

$$59 \div 7 = 8.42…$$

59 is a prime number!

Example Questions

$$19\div2=9.5$$

$$19 \div 3 = 6.333...$$

$$19 \div 5 = 3.8$$

$$19 \div 7 = 2.714…$$

19 is a prime number.

$$42\div2=21$$

$$42\div 3 = 14$$

$$42 \div 5 = 8.4$$

$$42\div 7 = 6$$

42 is not a prime number.