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 are willing to reduce the price, I will order 4 more pieces for every 1 yuan reduction." As much profit as before. How much is the cost of this product?
张先生向商店订购某种商品80件，每件定价100元．张先生向商店经理说："如果你肯减价，每减价1元，我就多订购4件．"商品店经理算了一下，如果减价5%，由于张先生多订购，仍可获得与原来一样多的利润．问这种商品的成本是多少元？
<gadget id="calculator">5 * 100</gadget>
<output>500</output>

<gadget id="calculator">100 - 80</gadget>
<output>20</output>

<gadget id="calculator">500 / 20</gadget>
<output>25</output>

<gadget id="calculator">100 - 25</gadget>
<output>75</output>

<result>75</result>