Draw a number line and show all numbers that are between 1 and 20 and have exactly 4 factors (or visualize, make it somewhat close to being a number line) (work with a number line all positives, 1 being smallest, 20 being biggest.

1 = 1 x 1 
1 factors

2 = 1 x 2
2 factors

3 = 1 x 3
2 factors

4 = 1 x 4
4 = 2 x 2
3 factors

5 = 1 x 5
2 factors

6 = 1 x 6
6 = 2 x 3
4 factors

7 = 1 x 7
2 factors

8 = 1 x 8
8 = 2 x 4
4 factors

9  = 1 x 9
9 = 3 x 3
3 factors

10 = 1 x 10
10 = 2 x 5
4 factors

11 = 1 x 11
2 factors

12 = 1 x 12
12 = 2 x 6
12 = 3 x 4
6 factors

13 = 1 x 13
2 factors

14 = 1 x 14
14 = 2 x 7
4 factors

15 = 1 x 15
15 = 3 x 5
4 factors

16 = 1 x 16
16 = 2 x 8
16 = 4 x 4
5 factors

17 = 1 x 17
2 factors

18 = 1 x 18
18 = 2 x 9
18 = 3 x 6 
6 factors

19 = 1 x 19
2 factors

20 = 1x 20
20 = 2 x10 
20 = 4 x 5
6 factors

So they are 6, 8, 10, 14 and 15 on the number line

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

The correct answer is addition