Three years ago, the age of a father was three times the age of his son. If the sum of their current ages is 70 years, what is the present age of the son?

19 years

20 years

25 years

None of these

Let son's current age be x and father's current age be y.

x + y = 70 ... (1)

Three years ago,

son's age = x - 3

father's age = y - 3

Given y - 3 = 3(x - 3)

y - 3 = 3x - 9

y = 3x - 6 ... (2)

Putting (2) in (1):

x + 3x - 6 = 70

4x = 76

x = 19