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. The original number is 39 . If the units digit and tens digit are y and x x + y = 12 10y + x = 10x + y + 54 equation 2 can be equal to y = x + 6 put y = x + 6 in equaiton 1 x + x + 6 = 12 2x = 6 x = 3 y = 3 + 6 y = 9 so the number is 39 .