Question

A car travels 200kms at a speed of 60km/hr and returns at speed of 40km/hr. What must be it's average speed

Answer 1

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

Hi..
Hope this helps u!!