A chicken and a rabbit have 100 feet, and a chicken and a rabbit have 40 feet. How many rabbits are there?
鸡和兔共有脚一百只，鸡和兔共有40只，有兔子多少只．
<gadget id="calculator">40 * 2</gadget>
<output>80</output>

<gadget id="calculator">100 - 80</gadget>
<output>20</output>

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

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<result>10</result>