Physics
posted by JP .
Car travels up hill at a constant speed of 50 km/h and returns down the hill at a constant speed of 65 km/h. Calculate the avg speed for the round trip in km/h.

Let's say the distance 2d is the total distance traveled. It is broken up into two parts. Time is distance divided by speed (km / km/hr = hr).
So, the average velocity v over the whole distance 2d can be found by adding the times for the two halves of the trip
2d/v = d/50 + d/65
2/v = 1/50 + 1/65
It's just a classic work problem in disguise
2/v = 115/3250
v = 2*3250/115 = 56.52 km/h
Just to check, say the trip was 100 km
at 50 km, that's 2 hrs
at 65 km, that's 1.538 hrs.
200 km at 56.52 km/h = 3.538 hrs, the sum of the uphill and downhill times.