Question

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?

Answer 1

For the same distance covered, the average speed is given by :

Avg speed = 2*a*b/(a+b), where a & b are respective speeds.

Avg speed = 2*84*56/(84+56) = 2*84*56/140 = 2*84*4/10 = 67.2 kmph

Alternatively, you can also do by: Avg speed = Tot dist/Total time
Avg speed = (778+778)/(778/84 + 778/56) = 1556/23.154 = 67.2 kmph

Hope this helps

Answer 2

Use eq. 2×v1 ×v2÷(v1+v2)

Where v1 and v2 are its velocities i.e.

84 and 56

Then,

2×84×56÷(56+84) =9408÷140

=67.2