a train accelerates from rest to a speed v m/s. it continues at this constant speed for a certain time and then decelerates uniformly to rest. if the average speed for the whole journey is 5v/6, show that the 4/5 of the whole distance is covered at a constant speed.

i cant work this out. i am trying to make use of the formulas average speed = total distance/total time and s=t(u+v)/2 . i am working these out with the letters but it aint working out