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 options.
A) 67.0 km /hr
B) 67.2 km /hr
C) 69.0 km /hr
D) 69.2 km /hr
E) None
Solution   Required average speed	=(2xy / x+y)km/hr   = 2 x 84 x 56 /(84 + 56)   = (2 x 84 x 85 /140)   = 67.2 km/hr.   Answer 67.2 km /hr
<result>B</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?