A deer is standing 80m in from the west end of a tunnel. The deer sees a train approaching from the west at a constant speed ten times the speed the deer can run. The deer reacts by running toward the train and clears the exit when the train is 20m from the tunnel. If the deer ran in the opposite direction it would be hit by the train at the Eastern entrance. How long is the tunnel?

• 180 m.
  
  If a train is at a distance of 20+y m. from west end, then 'y' is the distance covered by train when deer runs towards the west end & train is 20m. away from tunnel.
  
  The deer is at 80m. from the west end & if the total distance of tunnel=80+x
  
  If the speed of the deer is 'd' & train '10d' ,then
  
  (y+20+80+x)/(10d)= x/d , 9x-y=100 ---(i) &
  80/d= y/10d, y=800 ---(ii)
  Substituting y=800 in (i),x=100
  So length of the tunnel=80+x= 80+100= 180m
  
• 10 years agoHelpfull: Yes(0) No(0)