Here’s how it works:
Number – sum of digits = special number
10 – 1 = 9
11 – 2 = 9
12 – 3 = 9
13 – 4 = 9
14 – 5 = 9
15 – 6 = 9
16 – 7 = 9
17 – 8 = 8
18 – 9 = 9
19 – 9 = 9
20 – 2 = 18
21 – 3 = 18
22 – 4 = 18
and so on until
29 – 11 = 18
30 – 3 = 27
31 – 4 = 27
32 – 5 – 27
and so on until
39 – 12 = 27
40 – 4 = 36
41 – 5 = 36
and so on until
49 – 13 = 36
For any number between
50 and 59, the number – sum of digits = 45
60 and 69, the number – sum of digits = 54
70 and 79, the number – sum of digits = 63
I could keep going. But anyway, all those “special numbers” 9, 18, 27, 36, 45, 54, 63, etc. have the same symbol each time.
If he got yours "wrong" then you may not have paid careful attention to the symbol or your arithmetic.