Chickens and rabbits are in the same cage, with a total of 100 legs and 40 heads. Then how many rabbits are there?
鸡、兔同笼，共有100条脚，40个头．则兔有多少只．
<gadget id="calculator">4 * 40</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>

<gadget id="calculator">40 - 30</gadget>
<output>10</output>

<result>10</result>