A man of mass 60kg lifts a 15kg mass to the top of a building of height 10m in 5minutes. His efficiency is

A man who weighs 60 kg can pull a body that weighs 15 kg to the top of a structure that is 10 metres tall in 5 minutes. initially, the power is used to lift the body (performing work)

$$\begin{gathered} {\mathrm{P}}_{\mathrm{i}}=\frac{\mathrm{mgh}}{\mathrm{t}} \\ \Rightarrow {\mathrm{P}}_{\mathrm{i}}=\frac{(60+15)\times 10\times 10}{5\times 60}=\frac{7500}{300} \end{gathered}$$

When  reaching the top, the power is

$$\begin{gathered} {\mathrm{P}}_{\mathrm{f}}=\frac{\mathrm{mgh}}{\mathrm{t}} \\ {\mathrm{P}}_{\mathrm{f}}=\frac{15\times 10\times 10}{5\times 60}=\frac{1500}{300} \end{gathered}$$

The ratio of final power to initial power represents a person's efficiency.

$$\begin{gathered} \mathrm{\eta }=\frac{{\mathrm{P}}_{\mathrm{f}}}{{\mathrm{P}}_{\mathrm{i}}} \\ \mathrm{\eta }=\frac{{\displaystyle \frac{1500}{300}}}{{\displaystyle \frac{7500}{300}}}\times 100 \\ \mathrm{\eta }=\frac{1}{5}\times 100 \\ \mathrm{\eta }=20\% \end{gathered}$$

Hence the correct answer is 20%.