There’s the same symbol next to each number of the 9 times tables except 90
and 99. Every single 2-digit number that is added up, the total of it is subtracted
from the original number; the total will always be a multiple of 9.
Example:
Every single number from 10 to 19, 20 to 29 etc, the answer will be in the 9 times
tables. 10 to 19 = 9, 20 to 29 = 18.
10 = 1 = 9 20 = 2 = 18
11 = 2 = 9 21 = 3 = 18
12 = 3 = 9 22 = 4 = 18
13 = 4 = 9 23 = 5 = 18
14 = 5 = 9 24 = 6 = 18
As you can see, every time you add on 1 to the first number (e.g. 10, 11, 12…),
the second number would also have 1 added on, which means that the answer
would be the same because you made the numbers larger by the same amount
until you reach the next tens number.