Correction :

21 + 20 = 41	41 - 21 = 20
28 - 20 = 8	9 - 0 = 9
20 + 6 = 26	33 : 3 = 11
23 + 7 = 30	31 - 24 = 7
37 - 20 = 17	12 : 4 = 3
16 - 13 = 3	25 - 7 = 18
22 - 17 = 5	45 : 5 = 9
12 + 3 = 15	21 + 22 = 43
23 - 21 = 2	27 - 19 = 8
40 - 24 = 16	26 - 4 = 22