Correction :

6 + 12 = 18	11 + 5 = 16
31 - 8 = 23	18 + 18 = 36
20 + 15 = 35	15 + 0 = 15
23 - 23 = 0	27 - 17 = 10
22 + 17 = 39	25 - 12 = 13
23 - 8 = 15	19 + 9 = 28
26 - 2 = 24	28 - 20 = 8
16 + 18 = 34	34 - 20 = 14
30 : 3 = 10	10 + 13 = 23
30 - 7 = 23	7 + 18 = 25