Aptitude - Problems on Trains - Discussion

If faster train passes the slower train in 36 seconds then slower train passes the faster train in how many seconds?

If we consider relative speed = 46-36 = 10 (km/h), then at this speed the faster train needs to cover just the length of the slowest train (so we have just x not 2*x). Length of one train equals 100 m.

Also d1 = 460 m faster train in 36 s.

d2 = 360 m slower train in 36 s.

----x, d1.

---x, d2.

Let's suppose the length of train is x = 50 m, then d1-x = 460-50 = 410 m! = 360 m (distance covered by the slowest train).

So x = 100 m, length of one train.

Two trains are on parallel lanes then the one has to cross only 100 m or say x. So the correct answer is 100.

Why length of two train is needed? Please explain. The first train has to cross the length of single train only. Some body explain please.

There are two train A and B:

Let, the length of train A = X meter.

And, the length of train B also = X meter.

Then, the total length = (A+B) = (X+X) = 2X meter.

Relative speed = (46-36) km/hr = 10km/hr.

[Because, one train is faster then other].

Then, convert into m/sec = 10*5/18 = 25/9 m/sec.

As we know, Speed = Length/Time.

25/9 = 2X/36.

X = 50 [ by solving].

How did you get 5/18?

@Sandy.

I was confused too but I figured it out.

They use km/hr in the first place, and want to convert km/hr to m/sec.

Which is 100(m)/360(sec) (As we know that meter is 100 and sec is 60x60 in hour).

Therefore it will be 5/18 m/sec.

Hope you get it.

Speed = distance/time.

Here they have given that two trains are running parallel i.e., S1 & S2.

Relative speed = S1-S2 => 46-36 = 10kmph.

Now we have to convert kmph into m/sec.

So 10x5/18 = 25/9.

Speed = 25/9.

Time = 36 secs.

=>distance = speed x time.

=>25/9x36.

=>100 (length of the two trains).

Length of the each train = 50.

There first train is crossing the next train so when it crosses the fast train train will cross it's length and and second train length. So there relative speed and their combined lengths are taken.

We know for same direction, relative speed = (V1-V2).

Now, V1 = 46 Km/hr & V2 = 36 m/hr or we can say;

V1 = 46x5/18 m/s & V2 = 36x5/18 m/s.

Now, Relative Speed = Length or Distance/Time,

i.e (46x5/18) - (36x5/18) = length/36.

Take 5/18 common we get,

5/18(46-36) = length/36.

Length = 5/18x10x36, on solving we get, length = 100 meters.

But since length of trains is same.

Hence length of each train will be, length = 100/2 => length = 50 meters.