What number is missing to make this problem true?
8 + = 17
9
+ 10 = 19
9
+ 9 = 12
3
9 + = 16
7
+ 18 = 20
2
+ 10 = 11
1
15 + = 20
5
11 + = 15
4
+ 8 = 16
8
12 + = 13
1
13 + = 16
3
+ 8 = 10
2
7 + = 15
8
+ 6 = 16
10
11 + = 19
8
11 + = 18
7
1 + = 4
3
+ 3 = 17
14
+ 2 = 14
12
4 + = 9
5
+ 7 = 17
10
+ 1 = 1
0
17 + = 18
1
3 + = 18
15
0 + = 4
4
+ 4 = 20
16
7 + = 20
13
12 + = 19
7
15 + = 17
2