Aptitude - Problems on Trains - Discussion

S = D/S.
S = 120/12.
Therefore; S = 10 m/sec.
S = 10 × 18/5
And S = 36 kmph.

Where is 18/5 coming from?

speed=distance/time.
speed = 120/20,
then speed = 10 m/sec,
So, 10 x 18 /5.

Then the answer is 36 km/hr.

S=D/T.
S=120/12= 10m/sec,
=10*(18/5)=36km/hr.

Let the speed of a single train be x m/sec.
length= 120m.
Time to cross each other= 12 sec.

There is a formula like,

a+b/u+v = time to cross each other,
(i.e) 120+120/2x = 12,
240/2x = 12,
X = 40.

= 40 * (18/5)hkm/hr,
= 144.

The Speed of each train is,
= 144/2.
= 72.

Distance to be covered by each train to cross each other is 60m. (Half of the length of each train).

Then, speed= 60/12 = 5 m/s = 18 km/hr.

How 18/5 came?

How the speed of both trains same?

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).

(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.