There is a commodity whose purchase price in store A is 10% cheaper than that in store B. Store A sets the price according to 10% of the profit, and store B sets the price according to 20% of the profit. As a result, the price of store A is 21 yuan cheaper than that of store B. How much is the purchase price of store A?
有一种商品，甲店进货价比乙店进货价便宜10%．甲店按10%的利润来定价，乙店按20%的利润来定价，结果甲店的定价比乙店的定价便宜21元．问甲店的进货价是多少元？
<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">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 - (1/10)</gadget>
<output>9/10 = around 0.9</output>

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

<gadget id="calculator">(9/10) * (11/10)</gadget>
<output>99/100 = around 0.99</output>

<gadget id="calculator">(6/5) - (99/100)</gadget>
<output>21/100 = around 0.21</output>

<gadget id="calculator">21 / (21/100)</gadget>
<output>100</output>

<gadget id="calculator">100 * (9/10)</gadget>
<output>90</output>

<result>90</result>