The list price of an article is Rs. 900. It is available at two successive discounts of 20% and 10% . The selling price of the article is = ?

$$\eqalign{ & 20\% = \frac{1}{5},\,\,10\% = \frac{1}{{10}} \cr & {\text{MP}}\,\,\,\,\,\,{\text{SP}} \cr & \,\,\,{\text{5}}\,\,\,\,\,\,\,\,\,\,\,{\text{4}} \cr & \frac{{10\,\,\,\,\,\,\,\,\,\,9}}{{50\,\,\,:\,\,\,36}} \cr & 50{\text{ units}} = 900 \cr & {\text{36 units}} = \frac{{900}}{{50}} \times 36 \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 648 \cr} $$