Question

a father is three times as old as his son. 15 years hence he will be twice as old his son. what is the sum of their present ages?

Answer 1

Let the age of son be x years
Given that the age of the father  is 3 times of son
Therefore
father's age = 3x years

After 15 years
Age of son = x + 15 years
Age of father = 3x + 15 years

Also given after 15 years father's age becomes twice of son's age
Therefore
3x + 15 = 2 (x + 15 )
3x + 15 = 2x + 30 
x = 15 years

The age of son = 15 years
Age of father = 3x = 45 years

The sum of their ages = 15+45 = 60 years

Answer 2

Let the son's age be x.
Father's current age=3x
After 15 years,
Son's age= x+15
Father's age=2(x+15)
So,3x+15=2(x+15)
3x+15=2x+30
3x-2x=30-15
x=15
3x=15×3=45
Sum of their present ages=45+15
=60
Hope It Helps.