The age of a father, 9 years ago, was 4 times the age of his son. Nine years hence, the age of the father will be twice that of his son. Find the present age of the father.

Let the present age of father be x years and that of the son be y years.

Then, 9 years ago, father's age = x - 9 and son's age = y - 9 years

So, according to question,

x - 9 = 4 (y - 9) 

x - 4y = -27 ------------------(1)

Now, 9 years hence, father's age = x + 9 years and son's age = y + 9 years

Again, x + 9 = 2(y + 9)

x - 2y = 9 -------------------------(2)

On solving the equations (1) and (2), we get

x = 45 and y = 18

So, the present age of father is 45 years.