There are several chickens and rabbits in a cage. Counting from the top, there are 15 heads, and counting from the bottom, there are 50 feet. How many rabbits are there?
笼子里有若干只鸡和兔．从上面数，有15个头，从下面数，有50只脚．兔有多少只？
<gadget id="calculator">2 * 15</gadget>
<output>30</output>

<gadget id="calculator">50 - 30</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>