Question

On a 120 km track, a train travels the first 30 km at a speed of,30 km/hr. How fast must the train travels the rest so as to average 60km/hr for the entire trip?

Answer 1

45/2 kilometer per your

Answer 2

Hi.

Here is Your answer---

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Total Distance(d) = 120 km.

Initial Distance(d1) = 30 km.

Initial Speed = 30 km/hr.

Thus, Initial Time(t1) = Distance/Speed
                             = 30/30
                             = 1 hr.

Final Distance(d2)  = d - d1
                               = 120 - 30
                               = 90 km.

Let the Final Speed of the train be x km/hr.

Thus, Final Time(t2) = 90/x hr.

Total time = 1 + 90/x
                = (x + 90)/x.

Using the Formula,

  Average Speed = Total Distance/Total Time.
                       60 = 120/[(x + 90)/x]
                      60 = 120x /(x + 90)
                   x + 90 = 120x/60
                  x + 90 = 2x
                        x = 90

Thus the Final Speed of the Car be x km/hr.

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Hope it helps.


Have a nice day.