A car dealership has 20 bicycles and tricycles with 49 wheels in total. How many tricycles are there?
某车行有自行车和三轮车共20辆，总共有49个轮子．三轮车有多少辆．
<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<gadget id="calculator">49 - 40</gadget>
<output>9</output>

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

<gadget id="calculator">9 / 1</gadget>
<output>9</output>

<result>9</result>