Let x be the units digit of the number and y be the the tens digit.
x = y + 5
x - y = 5
x + ( 10x + y) = 79
x + 10x + y = 79
11x + y = 79
11 (x- y = 5)
(subtract from the first equation)
11x - 11y =55
- 11x + y = 79
0 - 12y = -24
-12y = -24
-12y / -12 = -24/-12
y = 2
x - y = 5
x - 2 = 5
x = 5 + 2
x = 7
The number is 27.