The unit price of a commodity is 40 yuan. If it is sold at a price of 50 yuan, the local market will be saturated after 500 units are sold. At the same time, market research shows that for every 1 yuan increase in the unit price of this commodity, the sales volume will decrease by 10 units. In order to earn the most profit, how much should the selling price be?
一种商品进货单价为40元，如果按50元的定价出售，卖出500个后本地市场就将饱和，同时市场调查表明，此种商品单价每涨1元，其销售量就减少10个．为了能赚取最多的利润，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>