Chickens and rabbits are in the same cage. There are 8 heads and 20 feet in total. How many chickens are there?
鸡兔同笼，共有8个头，20只脚，鸡有多少只．
<gadget id="calculator">8 * 4</gadget>
<output>32</output>

<gadget id="calculator">32 - 20</gadget>
<output>12</output>

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

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<result>6</result>