Question

The sum of the digits of a two-digit number is 12. The number formed by interchanging the digits is 54 more than the original number. What is the original number?

Answer 1

Let x and y be the value of the digits respectively and 10 x + y be the original number.
From the problem statement:
{ x + y = 12, 10 x + y - 54 = 10 y + x }
Solve the above for x and y.

{x = 9, y = 3}
93 is the original number.

Answer 2

you take the numbers and do math