It is known that there are chickens and rabbits in the cage, with a total of 72 legs and 30 heads. Do you know how many rabbits there are?
已知笼子里有鸡、兔两种动物，共72条腿，30个头，你知道有多少只兔吗？
<gadget id="calculator">30 * 2</gadget>
<output>60</output>

<gadget id="calculator">72 - 60</gadget>
<output>12</output>

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

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<result>6</result>