Answer to Question #76055 in Physics for Punyata

Question

A truck covers 30 km at a uniform speed of 30 km/hr. What should be its speed for the next 90 km if the average speed for the entire journey is 60 km/hr

Answer 1

The average velocity
v_ave=(l_1+l_2)/(t_1+t_2 )
t_1=l_1/v_1 ,t_2=l_2/v_2
Therefore
v_ave=(l_1+l_2)/(l_1/v_1 +l_2/v_2 )
60=(30+90)/(30/30+90/v_2 )
v_2=90 km/hr