A man walks on a straight road form his home to a market 2.5 km away with speed of 5 `(km)/(hr)`. Finding the market closed, he instantly turns and walks back home with a speed of 7.5 `(km)/(hr)`. The average speed of the man over the intervel of time 0 to 40 min is equal to

Options

A. 5 km/h
B. 25/4 km/h
C. 30/4km/h
D. 45/8km/h

Answer

D

Explanation

`"Average speed"=("Total distance covered")/("Total time taken")`
For onwards journey to market,
`t_(1)=("Distance")/("speed")=(2.5)/(5)=0.5h=(1)/(2)h=30` min
For backwards journey from market.
Time=(40-30)=10 min
distance=velocityxxtime`=(7.5xx10)/(60)=1.25` km
Average speed `=(2.5+1.25)/(40//60)=(3.75xx60)/(40)km//h`
`=(375xx6)/(4xx100)=(45)/(8) km//h`