First of all, since this is a math question, you should post this in the Mathematics section.
Secondly, call the smaller digit x - clearly, then, the larger digit is x+8. We know x is at least one, and x+8 is at most 9 (since otherwise x+8 would not be a digit). If x is greater than one, x+8 will be ten or more, but x cannot be less than one, so x must be one. As a result, our number has a tens digit of 1+8=9, so the number is 91.
