Correction :

40 - 19 = 21	36 - 18 = 18
19 + 3 = 22	8 - 2 = 6
29 - 9 = 20	11 - 8 = 3
9 + 2 = 11	6 : 6 = 1
7 + 12 = 19	18 - 18 = 0
4 - 2 = 2	12 + 18 = 30
20 - 6 = 14	23 - 13 = 10
39 - 17 = 22	10 + 11 = 21
30 - 11 = 19	38 - 16 = 22
13 + 18 = 31	11 + 9 = 20