The retail price of a certain commodity is 900 yuan per piece. In order to adapt to market competition, the merchant sells it at a discount of 10% off the retail price and a profit of 40 yuan. It can still make a profit of 50 yuan. What is the purchase price for each piece?
某种商品的零售价为每件900元，为了适应市场竞争，商家按零售价的9折并让利40元销售，仍可获利50元，求进价为每件多少元？
<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">900 * (9/10)</gadget>
<output>810</output>

<gadget id="calculator">810 - 40</gadget>
<output>770</output>

<gadget id="calculator">770 - 50</gadget>
<output>720</output>

<result>720</result>