There are 20 chickens and rabbits in total. There are 50 legs in total. How many chickens are there?
鸡和兔一共有20只，数一数腿有50条，其中鸡有多少只
<gadget id="calculator">20 * 4</gadget>
<output>80</output>

<gadget id="calculator">80 - 50</gadget>
<output>30</output>

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

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<result>15</result>