On June 1, the water level in front of the dam was at an altitude of 75 meters, and 15 days later, the Three Gorges Reservoir was impounded to an altitude of 135 meters. Calculate the average daily water level rise by how many meters?
6月1日，坝前水位在海拔75米高程，15天后，三峡水库蓄水至135米高程．算一算平均每天水位升高多少米？
<gadget id="calculator">135 - 75</gadget>
<output>60</output>

<gadget id="calculator">60 / 15</gadget>
<output>4</output>

<result>4</result>