The retail price of a certain commodity is 9900 yuan per piece. If the store sells at 80% of the retail price and can still make a profit of 10% (compared to the purchase price), what is the purchase price?
某种商品的零售价为每件9900元，若商店按零售价的80%降价销售仍可获利10%(相对于进货价)，则进货价为多少元．
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 + (1/10)</gadget>
<output>11/10 = around 1.1</output>

<gadget id="calculator">(4/5) / (11/10)</gadget>
<output>8/11 = around 0.727273</output>

<gadget id="calculator">9_900 * (8/11)</gadget>
<output>7_200</output>

<result>7_200</result>