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 ?

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