A shopkeeper offers 10% discount on the marked price of his articles and still makes a profit of 20%. What is the actual cost of the article marked Rs. 500 for him ?

$$\eqalign{ & {\text{Marked price of article }} \cr & = {\text{Rs}}{\text{. 500}} \cr & {\text{Selling price of article }} \cr & = {\text{90% }}\,{\text{of}}\,{\text{500}} \cr & = \frac{{90}}{{100}} \times 500{\text{ }} \cr & = {\text{Rs}}{\text{. 450}} \cr & {\text{Cost price of article }} \cr & = 450 \times \frac{{100}}{{120}}{\text{ }} \cr & = {\text{Rs}}{\text{. 375}} \cr} $$