Let the digit at the unit place = x. Then the digit at the tens place = 10 (9 - x) = 90 - 10x. Hence, the number = 90 - 10x + x = 90 - 9x. The new number often reverse, the digits = 10x (9 - x) = 9x + 9. Their difference is 9. So, 90 - 9x - 9x - 9 = 9, or 81 - 18x = 9, or 81 - 9 = 18x, or 72 = 18x, or ⇒ x = 4. So, the digit at the unit place is 4 and the digit at the tens place is 5. Hence, the number = 54.