The distance between two stations A and B is 778 km. A train covers th

The whole journey here represents a round-trip, since the train started from A and eventually came back to A.

In a round trip, the average speed is given by the formula,

Average Speed = \(\frac{2 a b }{ (a +b)}\), where

a = Speed of the object during the onward journey
b = Speed of the object during the return journey

The data about the distance between the two cities, is not needed, if we follow this approach to answer the question.

In this question, a = 84 and b = 56.
Substituting these values into the standard expression, we have,

Average speed = \(\frac{2 * 84 * 56 }{ (84 + 56)}\) =\(\frac{ 2 * 84 * 56 }{ (140)}\).

Simplifying, we have, Average speed = \(\frac{336 }{ 5}\).

67 * 5 = 335 and 68 * 5 = 340. Therefore, the value of \(\frac{336 }{ 5}\) has to be in between 67 and 68 which means the only value possible is 67.2

The correct answer option is D.

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