Aptitude - Problems on Trains - Discussion

Train A and Train B are each 120m long.

If train A is passing train B which is not moving OR if train A is passing train B which is also running and passing at the same speed, will train A pass train B quicker if train B is stopped or running and passing as well?

Train A will clearly pass quicker if Train B is running and passing as well, than it would if Train B was not moving.

If Train A and Train B take 12 seconds to pass if they are both moving, then it will take twice as long for Train A to pass Train B if Train B is not moving and so would take 24 seconds.

Therefore if Train A takes 24 seconds to cover 120 metres. (60m/12secs, 30m/6secs, 15m/3secs, 7.5m/1.5secs, 5m/1sec, 300m/60secs, 18000m/60mins, 18km/1hr).

Can you explain why this is wrong?

Surely it will take longer (double the time) for Train A to pass Train B (which is 120m long) if Train B was not moving and so it is actually covering 120m in 24 seconds. If they are both moving in opposite directions then to pass each other they are still travelling 120m each but in half the time (12 seconds).

Two vessels contains milk and water in the ratio 3:2 and 7:3 find the ratio in which the ratio contents of two vessels have to be mixed to get a mixture in which the ratio of the milk and water is 2:1?

Please solve this problem.

Why are we ignoring the relative speed?

If you assume that you are sitting on one train:

When crossing another train, distance to move to the end of opp train = 120meters,
distance to move so the end of your trains reaches other trains tail is 120meters

Total distance = 120 + 120 = 240 metres.
Time taken = 12sec.
Relative speed = v + v = 2v.

2v = 240/12 = 20,
=> v= 10metres/sec.
v km/hr = 10 * (1/1000)/(1/3600) = 3600 * 10/1000 = 36km/hr.

Distance = 120 m,

Time = 12 sec, s = ?

So, T = D/S,
=> 12 = 120/s.

s = 120/12,
s = 10m/sec.
Then,
s = 10 * 18/5.
s = 36km/hr.

Why can't we consider as x kmph?

Why the relative speed of the two trains is taken as 2x?

(120*18/5) = 432.

The length of train = Speed * Time.

120m = ? * 12 sec.

120/12 = 10 m/s.

So, 10*18/5 => 36 km/hr = Speed.

Question is speed of each train?

Length of each train is 120 m and speed is also same.

So Speed = Distance/Time.

= 120/12 m/s.

= 10 m/s.

= 10*18/5 km/hr.

= 36 km/hr [1 m/s = 18/5 km/hr].

Answer will be 36 km/hr.

@Vidyasagar and @Ygesh this answer is for you.

Speed = 10 m/s, but they are asking the answer in km/h, so you need to change it.

Now see, 1 m = 1/1000 km and 1 sec = 1/3600 hr.

So 1 m/s = 1/1000/1/3600 which means 3600/1000 = 18/5.

Therefore 1 m/s = 18/5 km/h.

I hope you got it. If not tell me I will explain in a much simpler way.