Final answer:
The water level decreases by x per hour even if we only observe the level for half an hour. Therefore, the rate of decrease per hour is x.
Step-by-step explanation:
The question asks us to determine the amount by which the water level decreases in one hour if it decreased by a certain rate between 6 AM and 6:30 AM. This time interval is half an hour. If we denote the decrease in water level per hour as x, then for half an hour, the water level would decrease by x/2. However, since the question prompts us to find the rate of decrease per hour, we do not need to divide x by any number; the rate of decrease per hour is still x.