An article is sold at a gain of 15% . Had it been sold for Rs. 27 more, the profit would have been 20% . The cost price of the article is = ?

$$\eqalign{ & {\text{Let cost}}\,{\text{price}}\,{\text{be}}\,x \cr & {\text{profit}} = \frac{{15x}}{{100}} \cr & {\text{If}}\,{\text{it}}\,{\text{was}}\,{\text{sold}}\,{\text{for}}\,{\text{Rs}}{\text{.}}\,27\,{\text{more,}} \cr & {\text{profit}} = \frac{{20x}}{{100}} \cr & \frac{{20x}}{{100}} - \frac{{15x}}{{100}} = 27 \cr & \frac{{5x}}{{100}} = 27 \cr & \frac{x}{{20}} = 27 \cr & \therefore x = 540 \cr} $$