A car travels up a hill at a constant speed of 45 km/h and returns down the hill at a constant speed of 70 km/h. Calculate the average speed for the round trip.
let the distance uphill, and downhill be x miles each way
time uphill = x/45
time downhill = x/70
total time = x/45 + x/70 = (115x/3150)
total distance = 2x
avg speed = 2x/(115x/3150)
= 2x(3150/(115x)) = 54.78 mph