You can put this solution on YOUR website! 11.
For the difference between two real numbers to be 0, the numbers have to be must be equal. Therefore, for the digits of a two digit number to have a difference of zero, the digits must be equal. This number is less than 20 and greater than 10.
Therefore, the tens place must have a one in it. Since the tens place is 1, then the ones place must also be 1. So, we come to the answer "11."