Question

A car travels from A to B at a speed of 20 kmh–1 and returns at a speed of 40 kmh–1. The average velocity of the car for the whole journey is

Answer 1

This question is very simple.

Average speed = Total Distance / Total Time Taken

But we have only speeds S1 and S2

S1 = 40 km/hr

S2 = 60 km/hr

So now let us assume that the distance between a and b will be 120 km ( LCM of 40 and 60 )

D1 = 120 km

D2 = 120 km

Total Distance = D1 + D2 = 120 + 120 = 240 km

Now

T1 = D1/S1

T1 = 120/40 = 3 hr

T2 = D2/S2

T2 = 120/60 = 2 hr

Total Time = T1 + T2 = 3 + 2 = 5 hr

Now

Average Speed = Total Distance / Total Time = 240 / 5

Average Speed = 48 km/hr

Edit: 28 July 2017

You can also solve this que using a simple formula

Avg Speed = 2xy/x+y

x = 40 and y = 60

Avg Speed = 2 x 40 x 60/100

Avg Speed = 48 km/h

Answer 2

In the question Average Velocity is asked

Avg. Velocity = Total Displacment / Total Time

∵ Car goes from A⇒B then B⇒A Displacement = 0

∴ Average Velocity = 0 kmh⁻¹