Question

A Father is four times as old as his son. The age of the father at the time of the birth of his son was 30. Find the age of his son

Answer 1

Let, the present age of father be $x$ years and that of his son be $y$ years.
As per condition 1 ie., Father is four times as old as his son.
The equation becomes,
$x=4y$

or
$x-4y=0$
As per condition 2 i.e., The age of the father at the time of the birth of his son was 30.
The equation becomes,
$x-y=30$