Correction :

12 : 3 = 4	20 : 4 = 5
23 + 5 = 28	36 - 15 = 21
19 + 4 = 23	90 : 9 = 10
9 + 19 = 28	20 - 2 = 18
11 + 20 = 31	14 - 4 = 10
14 - 8 = 6	4 × 3 = 12
13 + 8 = 21	21 + 14 = 35
34 - 20 = 14	4 + 4 = 8
23 + 2 = 25	19 - 3 = 16
5 + 18 = 23	5 + 9 = 14