Question

A body cover one third of its journey with speed u next third with speed with v and the last third with speed w what is average speed

Answer 1

Let, S be the total distance of the journey. 

It travels S/3 at speed u. Time taken is = S/3u 

Next S/3 at speed v. Time taken is = S/3v 

Next S/3 at speed w. Time taken is = S/3w 

So, 

Total time of travel = S/3u + S/3v + S/3w = S(vw+wu+uv)/3uvw 

Average speed = Total distance/total time = S/[ S(vw+wu+uv)/3uvw] 

=> v_av = 3uvw/(vw+wu+uv)