Correction :

18 + 3 = 21	21 + 3 = 24
20 - 16 = 4	25 - 9 = 16
20 - 18 = 2	47 - 24 = 23
5 + 4 = 9	39 - 23 = 16
15 + 11 = 26	50 : 10 = 5
21 + 5 = 26	32 - 15 = 17
20 - 12 = 8	11 × 3 = 33
19 + 2 = 21	10 - 9 = 1
10 + 9 = 19	27 - 18 = 9
17 + 2 = 19	23 - 11 = 12