Aptitude - Problems on Trains - Discussion

Its very simple.

Train A start at 7 AM & B start at 8 AM. A travels 20 km in 1 hr ie, distance between A and B will be.

90 Km at 8 AM.

Relative speed = 20+25 = 45Km/hr.

Distance = 90km.

Time = 90/45 = 2 hr.

Total time = 2+1 (that was travelled by train A between 7 to 8 AM. ).

Answer will be 7+3 = 10 AM.

Who ever doesn't understand why they have taken (X-1). Please see the below example.

Length of the track between starting position of train A starts at 7A.M and B Starts at 8A.M is 200 km.

Let's say A train travels with 20 kmph for 5 hours.

Distance covered by train A is = 100 km.

Train B Travels with 25 kmph for 4 hrs.

Distance covered by train B is =100km.

A train => 7+5=12 noon.

B train => 8+4=12 noon.

If we consider 7 as X then 8 should become (X-1) when we compare with the destination time.

Train A Travel in 1 hour 20 km at 7 am.
They both started together 8 am.
That's why 110-20 = 90km Remaining,

Relative Speed = (20+25) = 45 km/hr.

Time = (Distance/Speed).

= (90/45)

= 2 Hours

That's why 8am + 2 hours = 10 am.

The answer of this question is incorrect if the B train start at 8am and by the speed 25km\hr then it will cover the distance of 50km in next 2 hours but the train A will cove the distance of 60km in next 3 hour by the peed of 20km\hr.

So both can be meet at 10 a.m.

@Joicy.

The train B starts an hour later than the train A. Thus the train time A is assumed as x (started at 7 am) and the train B starts at time 8 am which is later an hour of time x hence its x-1. Hope its clear to you now.

Why (X-1) term is used?

Dear Arun,
Because train is started 1 hours late, thats why x-1

Why we use x-1 not x+1?

Its Simple in another way that Total Distance was 110KM.... A start at 7 AM at the speed of 20 KM in 3 Hrs he will cover 60KMS whereas B start at 8 AM @ speed of 25 KM/Per Hour in 2 HRS he will covered 50 KMS that means 7+3=10AM

Thanks yuvraj.