The single discount equivalent to the discount series of 20%, 10% and 5% is = ?

Answer: Option B

Single equivalent discount for 20% and 10%
$$\begin{array}{rl}& =20+10-\frac{20\times 10}{100}\\ & =30-2\\ & =28\%\end{array}$$
Single equivalent discount for 28% and 5%
$$\begin{array}{rl}& =28+5-\frac{28\times 5}{100}\\ & =33-1.4\\ & =31.6\%\end{array}$$