Xiaoming and his mother went to the leather shoes wholesale station to order 80 pairs of leather shoes with a price of 100 yuan. His mother said to the wholesaler: "If you are willing to lower the price, then for every 1 yuan you drop, I will order 4 more pairs." The wholesaler quickly He agreed, because he had calculated that if the price was reduced by 5%, Xiaoming's mother would order more, and the profit would be 100 yuan more than before. Children, do you know how much this kind of leather shoes cost?
小明和妈妈到皮鞋批发站订购发价为100元的真皮皮鞋80双，妈妈对批发商说："如果你肯降价，那么你每降1元，我就多订购4双．"批发商很快就同意了，原因是他经过计算的，若降价5%，则由于小明的妈妈多订购，获得的利润反而比原来多100元．小朋友，你知道这种皮鞋的成本是多少元吗？
<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 - 100 - 400 - 100</gadget>
<output>-6_600</output>

<gadget id="calculator">8_000 + (-6_600)</gadget>
<output>1_400</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_400 / 20</gadget>
<output>70</output>

<result>70</result>