The sales price of a product is 900 yuan per piece. In order to participate in market competition, the store sells it at a discount of 10% off the selling price and then sells it at a profit of 40 yuan, which can still make a profit of 10%. How much is the purchase price of this product?
某商品的销售价格每件900元，为了参加市场竞争，商店按售价的九折再让利40元销售，仍可获利10%，此商品的进价为多少元？
<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">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">770 / (11/10)</gadget>
<output>700</output>

<result>700</result>