There are 20 chickens and rabbits in a cage. They have 64 legs. How many chickens are there?
笼中共有20只鸡和兔，它们有64只脚，其中鸡有多少只．
<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<gadget id="calculator">64 - 40</gadget>
<output>24</output>

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

<gadget id="calculator">24 / 2</gadget>
<output>12</output>

<gadget id="calculator">20 - 12</gadget>
<output>8</output>

<result>8</result>