After the bus arrives at the station, 15 people get off and 3 people get on the bus. At this time, there are 30 people on the bus. How many people are there on the bus?
大客车到站后下车15人，上车3人，这时车上有30人，原来大客车上有多少人．
<gadget id="calculator">15 - 3</gadget>
<output>12</output>

<gadget id="calculator">30 + 12</gadget>
<output>42</output>

<result>42</result>