Answer:
0.3 ft/h
Step-by-step explanation:
At hour 18, the depth is 21 ft.
At hour 9, the depth is 18 ft.
Then the average rate of change is ...
(change in depth)/(change in time) =
(21 ft -18 ft)/(18 h -9 h) = (3 ft)/(9 h) = 1/3 ft/h ≈ 0.3333 ft/h ≈ 0.3 ft/h
