There are chickens and rabbits in the same cage, with a total of 8 heads and 20 legs. Then how many chickens are there?
有鸡兔同笼，共8个头，20条腿．则鸡有多少只．
<gadget id="calculator">8 * 2</gadget>
<output>16</output>

<gadget id="calculator">20 - 16</gadget>
<output>4</output>

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

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

<gadget id="calculator">8 - 2</gadget>
<output>6</output>

<result>6</result>