Answer: 63
Explanation:
If the sum of the digits is nine, we know our options are:
0 and 9
1 and 8
2 and 7
3 and 6
4 and 5
When you reverse the numbers, you decrease the value by 27.
To find this, we can subtract the small of the numbers from the bigger and see what subtracts to 27.
90 - 09 = 81
81 - 18 = 73
72 - 27 = 45
63 - 36 = 27
54 - 45 = 9
We see that the number must be 63.