How many prime numbers are between 4 and 20?

Answer 1

"Prime" numbers are ones that cannot be evenly divided by any other number.

So the following numbers are prime;

5

7

11

13

17

19

The non-prime numbers and their prime factors are:

4 = 2*2

6 = 2*3

8 = 2*2*2

10 = 2*5

12 =2*2*3

14 =2*7

15 =3*5

16 =2*2*2*2

18 = 2*3*3