Correction :

19 + 12 = 31	21 - 2 = 19
28 - 17 = 11	42 - 21 = 21
20 + 4 = 24	29 - 13 = 16
14 + 19 = 33	29 - 23 = 6
23 + 3 = 26	23 + 21 = 44
12 + 6 = 18	0 + 0 = 0
35 - 11 = 24	7 × 8 = 56
38 - 21 = 17	50 : 5 = 10
19 - 19 = 0	47 - 24 = 23
16 + 22 = 38	5 × 9 = 45