Chickens and rabbits are in the same cage. There are 30 heads and 100 feet. How many chickens are there?
鸡兔同笼，共有30个头，100只脚，鸡有多少只．
<gadget id="calculator">30 * 2</gadget>
<output>60</output>

<gadget id="calculator">100 - 60</gadget>
<output>40</output>

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

<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">30 - 20</gadget>
<output>10</output>

<result>10</result>