Question 977923
let the tens digit =x
let the ones digit= y
The number is 10*x+y;; 10*y+x is the reverse
10x+y+9=10y+x
9x+9=9y.  Divide everything in the equation by 9.
x+1=y
Their sum is 15.
x+x+1=15
2x=14
x=7
y=8

The number is 78
Add 9, and you get 87.