A trader wishes to gain 20% after allowing 10% discount on the marked price to his customers. At what percent higher than the cost price must he mark his goods ?

Answer: Option B

Let the cost price of goods = Rs. 100
Selling price of goods
= 120% of 100
= Rs. 120
Mark price of goods
$$\begin{array}{rl}& \text{ = 120}\times \frac{100}{90}\\ & =\frac{400}{3}\end{array}$$
Difference of Mark price and Cost price
$$\begin{array}{rl}& \text{ = }\frac{400}{3}-100\\ & \text{Difference }\%\\ & =\frac{\frac{100}{3}}{100}\times 100\\ & =\frac{100}{3}\%\\ & =33\frac{1}{3}\%\end{array}$$