Hint: In the given problem, we need to understand the relation between the ages of father and his son.
Let the present age of son = x years
Then, The present age of father = 3x years
After 12 years, their ages will be
Age of son = x + 12 years
Age of father = 3x + 12 years.
From the given information it is known that, father age will be twice the age of his son after 12 years, Then we can write
3x + 12 = 2 (x + 12)
On simplification, it will be
x = 12.
Father’s age is 3x = 3(12) = 36.
$\therefore $ The present age of the son is 12 years and his father's age is 36 years.
We have to clearly understand the given information, the relation between the father and his son's age at present and after 12 years, to represent them in mathematical variables. After getting everything in variables we can easily simplify the variables to get the desired values.