Distance between two stations A and B is 778 km. A train covers the journey from A to B at
84 km per hour and returns back to A with a uniform speed of 56km per hour. Find the
average speed of the train during the whole journey?
If a car covers a certain distance at x kmph and an equal distance at y kmph. Then,
the average speed of the whole journey =
2
x
y
x
+
y
kmph.
By using the same formula, we can find out the average speed quickly
average speed =
2
×
84
×
56
84
+
56
=
2
×
84
×
56
140
=
2
×
21
×
56
35
=
2
×
3
×
56
5
=
336
5
=
67.2
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