Correction :

21 - 15 = 6	8 × 5 = 40
0 + 18 = 18	15 - 5 = 10
19 + 10 = 29	15 + 16 = 31
2 × 9 = 18	9 × 10 = 90
9 + 24 = 33	6 × 5 = 30
33 - 9 = 24	9 × 11 = 99
22 + 16 = 38	36 - 24 = 12
2 + 24 = 26	36 - 20 = 16
28 - 9 = 19	28 - 11 = 17
24 - 2 = 22	28 - 5 = 23