CDS Model Paper 1 Mathematics

Let the unit's digit be y and the ten's digit be x.
Then, the number is 10x+y.
Interchanging the numbers, the new number is 10y+x.
Then, 10y+x=10x+y+18⇒9y−9x=18
⇒ y−x=2 and given x+y=8
Solving y=5,x=3
Then, the original number is 35 .