There are 40 chickens and rabbits in a cage with 100 legs. How many chickens are there?
笼中共有鸡和兔40只，有100只脚，鸡有多少只．
<gadget id="calculator">40 * 4</gadget>
<output>160</output>

<gadget id="calculator">160 - 100</gadget>
<output>60</output>

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

<gadget id="calculator">60 / 2</gadget>
<output>30</output>

<result>30</result>