Correction :

1 + 3 = 4	4 + 8 = 12
3 - 2 = 1	36 - 17 = 19
21 - 6 = 15	121 : 11 = 11
20 - 18 = 2	40 - 19 = 21
25 - 15 = 10	29 - 12 = 17
12 + 9 = 21	21 + 14 = 35
24 + 17 = 41	35 - 24 = 11
3 - 1 = 2	19 + 4 = 23
36 - 19 = 17	25 - 7 = 18
31 - 17 = 14	26 - 16 = 10