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

Answer: Option A

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