store has bought a batch of pens. If you sell 20 pens at a retail price of 7 yuan and sell 15 pens at a retail price of 8 yuan, you will earn the same amount of money. So how much is the purchase price of each pen?
商店进了一批钢笔，如果用零售价7元卖出20支与用零售价8元卖出15支所赚的钱数相同．那么每支钢笔的进货价是多少元？
<gadget id="calculator">7 * 20</gadget>
<output>140</output>

<gadget id="calculator">8 * 15</gadget>
<output>120</output>

<gadget id="calculator">140 - 120</gadget>
<output>20</output>

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

<gadget id="calculator">20 / 5</gadget>
<output>4</output>

<result>4</result>