speed on first trip --- x km/h
speed on return trip --- x-20
let the distance be d
time for first part = d/x
time for 2nd part = d/(x-20)
d/x = 2 ---> d = 2x
d/(x-20) = 3 ---> d = 3x - 60
3x-60 = 2x
x = 60
So the distance each way was 120 km
average speed = 240/5 = 48 km/h
Notice that the two speeds were 60 km/h and 40 km/h, and to get the average speed we CANNOT just average the two speeds.
alberto drove to the lake and back. the trip there took two hours and the trip back took three hours. he averaged 20 km/h faster on the trip there than on the return trip. What was alberto's average speed on the outbound trip?
2 answers
since distance = speed * time, if the outbound speed was x, then
2x = 3(x-20)
2x = 3(x-20)