There are 13 chickens and rabbits, and there are 40 legs, how many of them are chickens?
鸡和兔共13只，数一数腿有40条，其中鸡有多少只．
<gadget id="calculator">13 * 4</gadget>
<output>52</output>

<gadget id="calculator">52 - 40</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>