Owl: A river ferry leaves the dock at mile marker 2.0 and docks upstream at mile 5.0 after 24 minutes. Someone asks how fast it was going the moment the horn blew at departure.
You can compute an average speed for the whole crossing. That number is a summary — one ratio for one interval. It is not automatically what a GPS speed readout would have shown at t = 0.
Average rate of change is the secant's story: rise over run between two points. Instantaneous rate is the limiting local slope when the second point slides toward the first.
Look at the ferry numbers on the board. What can you compute right now, and what information are you still missing for the "horn blow" question?