Acar moves uphill at 40 km/h and then back downhill at 60 km/h. what is the average speed for the round trip?

Explanation:

Ok, the average speed can be calculate with the next equation:

  (1)

Basically the car cover the same distance "d" two times, but at different speeds, so:

and the total time would be the time t1 required to go from A to B plus the time t2 required to go back from B to A:

From basic physics we know:

so:

Using the previous information in equation (1)

Factoring:

  (2)

Finally, replacing the data in (2)

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