Question 1200739
Let x be the son's age.
s= x
f= 5x

4 years ago, the father's age would have been
f-4 and it also would have been 9(x-4).

This will give you the equation: f-4 = 9(x-4)
- Substitute 'f' for 5x as shown above to give you your final equation: 5x-4 = 9(x-4)

4x = 32
x= 8
 8 is the son's age now because it is equal to x and 5x8 is equal to the father's age now.
FINAL ANSWER: The father's current age is 40.