The altitude of A is +30 meters, the altitude of B is +20 meters, and the altitude of C is -10 meters. Which place is the highest, which place is the lowest, and how many meters higher is the highest than the lowest?
甲地海拔高度是+30米，乙地海拔高度是+20米，丙地海拔高度是-10米．哪个地方最高，哪个地方最低，最高的比最低的高多少米？
<gadget id="calculator">30 - (-10)</gadget>
<output>40</output>

<result>40</result>