Correction :

31 - 11 = 20	38 - 15 = 23
9 + 16 = 25	14 + 13 = 27
4 + 10 = 14	22 + 13 = 35
24 + 2 = 26	20 : 4 = 5
1 × 8 = 8	28 - 19 = 9
18 - 6 = 12	10 + 16 = 26
6 - 1 = 5	10 - 4 = 6
25 - 2 = 23	11 × 9 = 99
9 + 24 = 33	11 × 3 = 33
12 + 3 = 15	19 + 7 = 26