When a bus arrives at a stop, 15 people get off the bus, and the number of passengers left in the bus is twice the number of people who got off the bus. How many passengers were there in the bus?
一辆公共汽车到站时，有15人下车，留在车厢里的乘客人数是下车人数的2倍，车厢里原来有乘客多少人？
<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<gadget id="calculator">15 + 30</gadget>
<output>45</output>

<result>45</result>