Calculating average speed means dividing the average distance by the amount of time.
Appleton to Brockley: 50m/1h
Brockley to Cantham: 70m/1h
If Harry travels 70 mph, then this means it takes him 3 hours to reach Cantham, or 210/3. Let's look at the total distance: 50 + 210 = 260 miles. The the total time: 1 + 3 = 4 hours.
This leaves 260/4, which can be written as 260 miles every 4 hours, or just and average speed of 65 mph.
Hope this helped. Best of luck.