Mr. Zhang ordered 80 pieces of a certain product from the store, each priced at 100 yuan. Mr. Zhang said to the store manager: "If you reduce the price, I will order 4 more pieces for every 1 yuan less." same amount of profit. What is the cost price of this commodity.
张先生向商店订购某种商品80件，每件定价100元．张先生对商店经理说："如果你减价，每减价1元，我就多订购4件．"商店经理算了一下，如果减价5%，由于张先生多订购，仍可获得与原来同样多的利润．这种商品的成本价是多少元．
<gadget id="calculator">80 * 100</gadget>
<output>8_000</output>

<gadget id="calculator">5 * 4 * 100</gadget>
<output>2_000</output>

<gadget id="calculator">5 / 100</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">80 * (1/20) * 100</gadget>
<output>400</output>

<gadget id="calculator">5 * 4 * (1/20) * 100</gadget>
<output>100</output>

<gadget id="calculator">2_000 - 8_000 - 400 - 100</gadget>
<output>-6_500</output>

<gadget id="calculator">8_000 + (-6_500)</gadget>
<output>1_500</output>

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

<gadget id="calculator">20 - 80</gadget>
<output>-60</output>

<gadget id="calculator">80 + (-60)</gadget>
<output>20</output>

<gadget id="calculator">1_500 / 20</gadget>
<output>75</output>

<result>75</result>