Aptitude - Problems on Trains - Discussion

(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.

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

Why can't we consider as x kmph?

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.

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.

Why are we ignoring the relative speed?

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.

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).

Can anyone tell me that answer is asked in km\hr, but the answer given is s\m?

Why?

Let the speed of a singe train be x m/sec.
length= 120m.
time= 12 sec,

speed = length/time.
= 120/12.
= 10 m/s.

According to given question, they want speed in km/hr so we will convert it by formula (a*18/5)km/hr.

In our case a is 10m/s.
So
speed=(10*18/5)km/hr,
=36 km/hr.