A train covers half of its journey with a speed of 30m/s and other half with a speedof 40m/s. Calculate the average speed of the train during the whole journey.

Let the total distance it covers be 2x

Time taken to cover the first half of distance  t₁ = x/30

Time taken to cover the next half of distance  t₂ = x/40

Total time taken = t₁ + t₂

  = x/30 + x/40

  = 7x/120

Average speed = total distance/total time taken

  = 2x/(7x/120)

  = 240/7 ms^-1

 ^{= 34.3 m/s}