originally planned to buy 20 pairs of shoes at 100 yuan each, and bargained with the hawker. If it was 1 yuan cheaper, I would buy 5 more pairs, which turned out to be 4 yuan cheaper. After the hawker sold out, the profit was 80 yuan more than originally planned. How much does a pair of shoes cost?
原定买鞋子20双每双一百元，和小贩讨价还价，如果便宜一元，多买5双，结果便宜了4元．小贩卖完后一算，利润比原定多80元．问鞋子成本多少钱一双？
<gadget id="calculator">4 * 20</gadget>
<output>80</output>

<gadget id="calculator">4 * 5 * 4</gadget>
<output>80</output>

<gadget id="calculator">80 + 80 + 80</gadget>
<output>240</output>

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

<gadget id="calculator">20 - 20</gadget>
<output>0</output>

<gadget id="calculator">20 + 0</gadget>
<output>20</output>

<gadget id="calculator">240 / 20</gadget>
<output>12</output>

<gadget id="calculator">100 - 12</gadget>
<output>88</output>

<result>88</result>