It costs 116 yuan to buy 4 ducks and 2 roosters, and 188 yuan to buy 4 ducks and 6 roosters. How much does each duck cost?
买4只鸭子和2只公鸡要花116元，买4只鸭子和6只公鸡要花188元．每只鸭子要卖多少钱？
<gadget id="calculator">188 - 116</gadget>
<output>72</output>

<gadget id="calculator">6 - 2</gadget>
<output>4</output>

<gadget id="calculator">72 / 4</gadget>
<output>18</output>

<gadget id="calculator">18 * 2</gadget>
<output>36</output>

<gadget id="calculator">116 - 36</gadget>
<output>80</output>

<gadget id="calculator">80 / 4</gadget>
<output>20</output>

<result>20</result>