If the price of a product is increased by 20% on the basis of the purchase price, and then sold at a 10% discount, and the result is a profit of 40 yuan, what is the purchase price?
一件商品在进价的基础上提价20%，又以9折销售，结果获利40元，则进价是多少元．
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 + (1/5)</gadget>
<output>6/5 = around 1.2</output>

<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">(6/5) * (9/10)</gadget>
<output>27/25 = around 1.08</output>

<gadget id="calculator">(27/25) - 1</gadget>
<output>2/25 = around 0.08</output>

<gadget id="calculator">40 / (2/25)</gadget>
<output>500</output>

<result>500</result>