Number System Practice Test 1

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.