A man of mass 80 kg carrying a load of 20 kg walks up a stair case in 20s. If the number of steps is 40 and width and height of each step are 20 cm and 15 cm respectively. The efficiency of the man is 

Given,

mass of the man M = 80 kg

mass of the load m = 20kg

Efficiency,

$\mathrm{Efficiency}=\frac{\mathrm{useful} \mathrm{output} \mathrm{energy}}{\mathrm{total} \mathrm{input} \mathrm{energy}}$

The total work done by the man is 

${\mathrm{W}}_{\mathrm{ip}}=({\mathrm{m}}_{\mathrm{man}}+{\mathrm{m}}_{\mathrm{block}})\mathrm{gh}$

$\Rightarrow {\mathrm{W}}_{\mathrm{ip}}=(80+20)\mathrm{gh}=100\mathrm{gh}$

The output work done by the person is,

${\mathrm{W}}_{\mathrm{op}}={\mathrm{m}}_{\mathrm{block}}\mathrm{gh}=20\mathrm{gh}$

Thus the efficiency,

$$\begin{gathered} \mathrm{\eta }=\frac{{\mathrm{W}}_{\mathrm{op}}}{{\mathrm{W}}_{\mathrm{ip}}}=\frac{20\mathrm{gh}}{100\mathrm{gh}}=0.2 \\ \Rightarrow \mathrm{\eta }=20\% \end{gathered}$$

Hence the correct answer is 20%.