The cost of a commodity is 80 yuan each, if it is sold at 100 yuan each, 1000 pieces can be sold. When the price of each commodity increases by 1 yuan, the sales volume will decrease by 20. In order to earn the most profit, the selling price should be set at how many yuan each.
某商品成本为每个80元，如果按每个100元卖，可卖出1000个．当这种商品每个涨价1元，销售量就减少20个．为了赚取最多的利润，售价应定为每个多少元．
<gadget id="calculator">50 - 20</gadget>
<output>30</output>

<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<gadget id="calculator">100 + 15</gadget>
<output>115</output>

<result>115</result>