Correction :

18 - 13 = 511 + 20 = 31
15 + 4 = 1954 : 9 = 6
3 × 8 = 2417 + 13 = 30
8 - 8 = 06 + 20 = 26
15 + 21 = 3627 - 9 = 18
10 × 9 = 906 : 2 = 3
20 + 13 = 333 + 2 = 5
6 + 4 = 1023 - 20 = 3
4 + 15 = 1922 - 16 = 6
27 - 7 = 2035 - 22 = 13