A sum of money is sufficient to pay A's wages for 21 days and B's wages for 28 days. The same money is sufficient to pay the wages of both for ?

Answer: Option A

$$\begin{array}{rl}& \text{Let total money be Rs}\text{.}x\\ & \text{A’s 1day’s wages}=\text{Rs}\text{.}\frac{x}{21}\\ & \text{B’s 1day’s wages}=\text{Rs}\text{.}\frac{x}{28}\\ & \therefore \left(\text{A}+\text{B}\right)\text{’s 1 day’s wages}\\ & =\text{Rs}\text{.}\left(\frac{x}{21}+\frac{x}{28}\right)\\ & =\text{Rs}\text{.}\frac{x}{12}\end{array}$$
∴ Money is sufficient to pay the wages of both for 12 days.