The ratio of the number of boys and girls in a college is 7 : 8. If the percentage increase in the number of boys and girls be 20% and 10% respectively, what will be the new ratio?

Answer: Option C

Originally, let the number of boys and girls in the college be 7x and 8x respectively
Their increased number is (120% of 7x) and (110% of 8x)
$$\begin{array}{rl}& \Rightarrow \left(\frac{120}{100}\times 7x\right)\text{and}\left(\frac{110}{100}\times 8x\right)\\ & \Rightarrow \frac{42x}{5}\text{and}\frac{44x}{5}\\ & \therefore \text{The}\text{required}\text{ration}\\ & =\frac{42x}{5}:\frac{44x}{5}\\ & =21:22\end{array}$$