There are 100 chickens and rabbits, and 240 feet of chickens and rabbits. How many rabbits are there?
鸡与兔共100只，鸡和兔的脚共240只，问兔有多少只．
<gadget id="calculator">2 * 100</gadget>
<output>200</output>

<gadget id="calculator">240 - 200</gadget>
<output>40</output>

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

<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<result>20</result>