Aptitude - Problems on Trains - Discussion

When both trains start from same point and one passes another then we should take distance x m, not 2x metre. This condition is possible when fast train is behind the slow train.

Easiest process:

Let, length = x m.

Speed of 1st mvng train = 46 * (5/18) = 115/9 m/sec.

""slow"" = 36 * (5/18) = 10 m/sec.

Formula = time is taken by a first moving train to overtake (same direction) a slow moving train = (length of first moving train + length of slow moving train) / (speed of first moving train - speed of slow moving train).

Hence 36 = 2x / ((115/9) - 10) 2x is taken because both lenght are equal.

Hence, x = 50m.

With these explanations and doubts clear all my doubts.

Thanks to every one.

Where did you get 5/18? Explain me. It is confusing.

Sir, I am having some difficulties to solve a question. Some confusion is there. I need your help sir. The question is-.

Two trains are running at 40km/hr and 20 km/ hr respectively in the same direction. Fast train completely passes a man sitting in the slower train in 5 sec. What is the length of the fast train?

Sir the formulas is a+b/u-v.

Where a and b lengths and u and v are speeds.

My question is that: why we are not taking both the lengths. In solution, only fast train lengths is taking. Why sir. A/c to formula both the lengths are necessary.

The length of the train is equal.

So let it x.

Then applying the formula.

a + b/u - v = Time taken by the faster train passes the lower train.

x + x/46 - 36 = 36.

The speed of trains are in km/hr. So it have to convert in m/sec.

Firstly it subtract.

Because fraction value will come. So it difficult to multiply.

It's either subtract before or after.

Then, 46 - 36 = 10 km/hr.

10 * 5/18 = 25/9.

The answer will come 2x/25/9 = 36.

Then the 9 goes to multiply denominator.

9*2x/25 = 36.

Then for easy calculation 2x/25 = 36/9.

x = 50.

Can anyone tell me why only dividing by 36 why not by 46?

I think the answer should be 100 meters, because in question it is not given the starting positions of trains. So we have to assume they started from same point.

Now let the slower train is at rest then the faster train's speed would be 10 km/hr, and it cross the slower train (length) in 36 sec. So the LENGTH = 10km/hr*30 sec = 100 meters.

Train = ||.

Train 1 = |1| speed is 46 km.

Train 2 = [2] speed is 36 km.

*Diagram:

1. Imagine at (2:00 am) both are moving in parallel direction.

[2] (T1 haven't covered x distance).

|1| at (2:00:36 am).

|1| [2] (T1 covers first x distance).

# At (2:01:13 am).

|1| (T1 covers next x distance).

[2] ((Now t1 coverd 2x distance)).

That's how distance is 2x.

We are finding the relative speed.