When the purchased commodity with a unit price of 40 yuan is sold at 50 yuan, each profit is 10 yuan, but only 500 units can be sold. It is known that each price increase of this commodity by 1 yuan will reduce its sales volume by 10 yuan. indivual. In order to earn the most profit, how much should the selling price be.
将进货的单价为40元的商品按50元售出时，每个的利润是10元，但只能卖出500个，已知这种商品每个涨价1元，其销售量就减少10个．为了赚得最多的利润，售价应定为多少元．
<gadget id="calculator">50 - 10</gadget>
<output>40</output>

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

<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">50 + 20</gadget>
<output>70</output>

<result>70</result>