The average speed is calculated by dividing the total distance covered over the total time spent to cover that certain distance. From the given,
total distance = distance from Bristol to Cardiff + distance from Cardiff to Swansea
Substituting,
total distance = 72 km + 69 km = 141 km
Calculate first for the time spent to travel from Cardiff to Swansea.
time = 69 km / (60 km/h) = 1.15 h
Calculating for the total time,
time = (48 minutes / 60 minutes) + 1.15 h
time = 1.95 h
Substituting this to the equation for average speed,
average speed = 141 km / 1.95 h
= 72.3 km/h
Answer: 72.3 km/h