The speed of the bicyclist in miles per hour is the ratio of the total distance covered to the time spent.
The total distance
= 4.5 + 7.5
= 12 miles
The total time spent on the journey 8:am to 8:48am
=48minutes
Average speed is required in miles per hour, therefore, 48mins must be converted to its equivalent in hours
60 mins makes 1hour
48 mins will be 48/60 = 0.8hours
Recall that speed= distance/time
speed=12/0.8
speed = 15 miles per hour