Solve the math problem.
4 - 2 =
19
- 2
17 - 6 =
14
- 0
15
- 5
11
- 6
9
- 7
- 1
- 9
13 - 7 =
18 - 9 =
6 - 6 =
20
17 - 2 =
16
8 - 2 =
7 - 5 =
12 - 5 =
17 - 4 =
12 - 3 =
8
18
8 - 8 =