Question 315130
let x= once digit and y is tenth digit.
Then , number is 10y+x
So, if the digits interchange their places, then the number is 10x+y

now, 
xy=20 ....(i)

10y+x+9=10x+y
or 9x-9y=9
or x-y=1 ..(ii)

From (i) and (ii) , we get

x(x-1)=20=4X5
x=4 or 5 , but as x-y=1 , then x=5 and y=4

The number is 45.