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 ?

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