Harsha is 40 years old and Ritu is 60 years old. How many years ago was the ratio of their ages is 3 : 5 ?

$$\eqalign{ & {\text{Harsha = 40}} \cr & {\text{Ritu = 60}} \cr & \frac{{{\text{Harsha}}}}{{{\text{Ritu}}}} = \frac{{40}}{{60}} \cr} $$
∴ Let x years ago their ages ratio was 3 : 5
$$\eqalign{ & \Rightarrow \frac{{40 - x}}{{60 - x}} = \frac{3}{5} \cr & \Rightarrow 200 - 5x = 180 - 3x \cr & \Rightarrow 2x = 20 \cr & \Rightarrow x = 10 \cr} $$