A man walks on a straight road from his home to a market 2.5 km away with a speed of 5 km/h. Finding the market closed, he instantly turns and walks back home at a speed of 7.5 km/h. The average speed of the man over the interval of time 0 to 40 min. is equal to  

A man walks from his home to market with a speed of  $5km/h$. Distance $=2.5km$  and time $=\frac{d}{v}=\frac{2.5}{5}=\frac{1}{2}hr$  and he returns back with speed of  $7.5km/h$ in rest of time of 10 minutes.
Distance $=7.5\times \frac{10}{60}=1.25km$
So, Average speed$=\frac{Totaldis\tan ce}{Totaltime}$
$=\frac{(2.5+1.25)km}{(40/60)hr}=\frac{45}{8}km/hr$