Problem Solving

At present the age of father is triple the age of son. After nine years the age of the son is half of the age of the father. What was the age of father when the son was born?

let the father's present age be "f" and the son's present age be "s".
so f = 3s (At present the age of father is triple the age of son)
and s + 9 = (1/2)(f + 9)
(Since After nine years the age of the son is half of the age of the father)

=>s + 9 = (1/2)(f + 9)
=>2s + 18 = f + 9
=>2s + 18 = 3s + 9 (Since f = 3s)
=> s = 9 Years.
so f = 27 years.

Now we want "What was the age of father when the son was born"
This means the age of the father 9 years back and that is 18 years.