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?
Choose the correct choice from the following choices.
A) 67.8  B) 67.4  C) 67.2  D) 67.6  E) 67.0
If a car covers a certain distance at x kmph and an equal distance at y kmph. Then,   average speed of the whole journey = (2xy)/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   =3365=67.2   Answer is 67.2.
<result>C</result>
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?