At what percent above the cost price must a shopkeeper mark his goods so that he gains 20% even after giving a discount of 10% on the marked price ?

Answer: Option C

Let Cost price = Rs. 100
Then,
Selling price = Rs. 120
Let marked price be Rs. x
Then,
90% of x = 120
$$\begin{array}{rl}& \Rightarrow x\text{ = }\left(\frac{120\times 100}{90}\right)\\ & =133\frac{1}{3}\\ & \therefore \text{Marked price}\\ & =\text{33}\frac{1}{3}\%\text{ above C}\text{P}\end{array}$$