When a bus departs from the starting station, there are some passengers on the bus. When we arrived at the experimental primary school station, 15 people got off first, and then 12 people got on. At this time, there were 40 people on the bus. How many people are on board when departing from the starting station?
一辆公共汽车从起点站开出时，车上有一些乘客．到了实验小学站，先下了15人，又上了12人，这时车上共有40人．从起点站出发时车上有多少人？
<gadget id="calculator">40 - 12</gadget>
<output>28</output>

<gadget id="calculator">28 + 15</gadget>
<output>43</output>

<result>43</result>