A bus got off at Daqiao Station, 7 people got off, and 8 people got on. Now there are 39 people on the bus. How many people were on the bus?
一辆公共汽车到大桥站有7人下车，又上来8人，现在车上一共有39人，车上原来有多少人．
<gadget id="calculator">7 - 8</gadget>
<output>-1</output>

<gadget id="calculator">39 + (-1)</gadget>
<output>38</output>

<result>38</result>