@Akintola also gets this

Let the first digit be x
Let the second digital be y
Therefore the number is 10x+y
When reversed the number is 10y+x

(10x+y)-(10y+x) =18>>>>i
x+y=4>>>>ii

From eqn i
10x-x-10y-y=18
9(x-y)=18
Divide both side by 9
x-y=2>>>>iii

Solve eqn ii and iii simultaneously

x+y=4>>>ii
x-y=2>>>iii
Add ii and iii

x+x+y-y=4+2
2x=6
Divide both side by 2
Therefore x=3

Sub x=3 into eqn ii
3+y=4
y=4-3
y=1

Therefore the numbers are 10(3)+1 or 10(1)+3
31 or 13

The number is 31 or 13