There are 20 tricycles and bicycles in the parking lot, and there are 42 wheels in total. How many bicycles are there?
停车场里有三轮车和自行车共20辆，共有42个轮子，自行车共有多少辆．
<gadget id="calculator">2 / 1</gadget>
<output>2</output>

<gadget id="calculator">20 - 2</gadget>
<output>18</output>

<result>18</result>