Let any two digit number, AB be written as:

AB = 10 x A + B.

The addition of the 2 digits is then A + B

Therefore: 10 x A + B MINUS A+B Eguals 9xA.

Note that no matter what the two digit number the units digit (B) is cancelled or is irrelevant. The answer is 9 times the second (or tens) digit.

If you picked any two digit number between 50 and 59 (inclusive), the answer is 45

Also, note that the symbol is the same for every number that is a multiple of 9 (9, 18, 27, 36, 45, etc)