Question

On a 60km straight road a bus travels the 30km with a uniform speed of 30km/h. How much fast the bus travels the next 30km. So as to have average speed of 40km/h for the entire trip?

Answer 1

we know that,    
                         av. speed=total distance/total time---------------------(1)
  av. speed=40 km/hr
  total distance=60 km/hrs
  let take t₁ as time taken for first 30 km and t₂ for last 30 km
  speed =distance/time , time=distance/speed-------------------------------(2)
                           t₁=30/30=1 hrs
putting in formula (1) 
                         
                          40=60/1+t₂
                          1+t₂=60/40
                          1+t₂=3/2
                              t₂=3/2-1=3-2/2=1/2 hrs
putting value of t₂ in formula----------(2)
        1/2=30/required speed
required speed=30×2=60 km/hr