A single discount equivalent to discount series 20% , 20% and 10% is = ?

Answer: Option C

Single equivalent discount for 20% and 20%
$$\begin{array}{rl}& =20+20-\frac{20\times 20}{100}\\ & =40-4\\ & =36\%\end{array}$$
Single equivalent discount for 36% and 10%
$$\begin{array}{rl}& =36+10-\frac{36\times 10}{100}\\ & =46-3.6\\ & =42.4\%\end{array}$$