The store has bought a batch of pens, and the profit of selling 20 pens at a retail price of 10 yuan is the same as selling 15 pens at a retail price of 11 yuan. How much is the purchase price of this batch of pens?
商店进了一批钢笔，用零售价10元卖出20只和用零售价11元卖出15支的利润相同．这批钢笔的进价是每支多少钱？
<gadget id="calculator">10 * 20</gadget>
<output>200</output>

<gadget id="calculator">11 * 15</gadget>
<output>165</output>

<gadget id="calculator">200 - 165</gadget>
<output>35</output>

<gadget id="calculator">20 - 15</gadget>
<output>5</output>

<gadget id="calculator">35 / 5</gadget>
<output>7</output>

<result>7</result>