Chickens and rabbits are in the same cage, with 40 legs and 15 heads. How many chickens are there?
鸡兔同笼，脚40只，头15个，鸡有多少只．
<gadget id="calculator">15 * 4</gadget>
<output>60</output>

<gadget id="calculator">60 - 40</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>