Chickens and rabbits are in the same cage. There are 25 heads and 80 feet. How many chickens are there?
鸡兔同笼，有25个头，80只脚，鸡有多少只．
<gadget id="calculator">25 * 4</gadget>
<output>100</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>