The current ages of Sonali and Monali are in the ratio 5 : 3. Five years from now, their ages will be in the ratio 10 : 7. Then Monali's current age is = ?

Answer: Option C

Let,
⇒ Sonali's age = 5x
⇒ Monali's age = 3x
According to the question,
$$\begin{array}{rl}& \Rightarrow \frac{5x+5}{3x+5}=\frac{10}{7}\\ & \Rightarrow \frac{x+1}{3x+5}=\frac{2}{7}\\ & \Rightarrow 7x+7=6x+10\\ & \Rightarrow x=3\end{array}$$
⇒ So, Monali's present age = 3x
= 3 × 3 = 9 years