Chickens and rabbits are in the same cage. There are 30 heads on the top and 80 legs on the bottom. How many chickens are there?
鸡兔同笼，上有30头，下有80条腿，其中鸡有多少只．
<gadget id="calculator">30 * 2</gadget>
<output>60</output>

<gadget id="calculator">80 - 60</gadget>
<output>20</output>

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

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

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

<result>20</result>