There are 20 bicycles and tricycles with 52 wheels in total. How many bicycles are there?
有自行车和三轮车共20辆，共有52个轮子，自行车多少辆．
<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<gadget id="calculator">52 - 40</gadget>
<output>12</output>

<gadget id="calculator">3 - 2</gadget>
<output>1</output>

<gadget id="calculator">12 / 1</gadget>
<output>12</output>

<gadget id="calculator">20 - 12</gadget>
<output>8</output>

<result>8</result>