56. Distanc e 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?
A. 67.0 km /hr B. 67.2 km /hr
C. 69.0 km /hr D. 69.2 km /hr
Answer:
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.
