Chickens and rabbits are in the same cage. There are 21 heads on the top and 66 feet on the bottom. How many chickens are there?
鸡兔同笼，上有21头，下有66足，有多少只鸡．
<gadget id="calculator">21 * 4</gadget>
<output>84</output>

<gadget id="calculator">84 - 66</gadget>
<output>18</output>

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

<gadget id="calculator">18 / 2</gadget>
<output>9</output>

<result>9</result>