Solving questions on two digit numbers

The general way of expressing any two digit no is 10 x+y (where y is unit place digit and x is tens place digit)

Question. The sum of 2 digits of a number is 9 if the digits are reversed the number is increased by 63. Find the number.

Solution: Let the digit at unit place be x, so the digit at tens place be 9-x
The number = 10(9-x)+x

According to the question:
[10x+(9-x)] - [10 (9-x) + x ] = 63
9x+9 - (90-10x+x) = 63
9x + 9 -90 + 9x = 63
18x - 81 = 63
18x = 144
x = 8