Posted by **JP** on Thursday, September 22, 2011 at 10:13pm.

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.

- Physics -
**Steve**, Friday, September 23, 2011 at 3:52pm
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.

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