Let x = smaller number.
So for this, our larger number equation is 10x, and to find what x is, we have to set it to 81. Our equation will be

Firstly, combine like terms:

Next, divide 9 on both sides of the equation, and your answer should be
.
Now that we know that the smaller number is 9, just multiply that by 10 to get 90.
And in short, the smaller number is 9 and the larger number is 90.
If x represented the smaller number, then the larger number is 10x.