Aptitude - Problems on Trains - Discussion

When two train s of a length a and b, of speed u, v m/sec.they move to cross each other. Time taken by the two trains cross each other is a+b(u+v). Because of these formula we have find the length of trains. I hope understood these problem.

Hai every body,
The speed of one train = x, Speed of another train = y.
We know that speed = length/time || length = speed*time.

So we find the length of the first train = x(speed)*27(time).

The length of the second train = y(speed)*17(time).

Time taken by both train to cross(total time) = 23.

Total length = (length of the first train)+(length of the second train) = (x*27)+(y*17).

Total speed = (speed of first train) + (speed of the second train) = x+y.
Total time = total length/total speed.

23 = [(x*27)+(y*17)]/(x+y).
27x+17y = 23(x+y).
27x+17y = 23x+23y ||27x-23x = 23y-17y || 4x = 6y ||x/y = 3/2.

@Faty how can you say total speed is (x+y). Duh the train are moving in different direction. So their total speed need to be (x-y) according to your point of view.

@Hardik Mistry : When trains move in opposite direction their speeds are added, but when they go in the same direction their speeds are subtracted. Imagine you are going on road by bike and another bike is coming from front (i. e opposite direction) , you feel that the other bike is coming faster. But if it comes from back and tries to overtake you (i.e same direction) then it overtakes you slowly as compared to its speed.

27x + 17y/(x+y) = 23;

27x + 17y = 23x + 23y;

4x = 5y;

x/y = 5/4.

First we are doing KM Hr to MS.
Then Formula is a*5/18.

Then,
45*5/18=75/6 MS.

Distance = Speed*Time.
= 75/6*30.
= 375.

Distance = 375-130.
= 245.

Let Train A distance = 27x Train A speed = x.

Train B distance = 17y Train B speed = y.

Time = distance/speed.

23 = 27x + 17y / x + y.

x/y = 3/2 ----> Answer.

:) The simple formula use for this problem is S = v*t.

Hi,

Nice explanation for why we need to.

1. Add the both trains speed which were running in opposite Directions with a reference point.

2. Subtract the both trains speed which were running in Same direction with a reference point.

http://en. Wikipedia. Org/wiki/Relative_velocity.

Relative speed of train = x+y;

Train 1 dis = 27x;
Train 2 dis = 17y;

Formula(speed = distance/time).

So x+y = (27x+17y)/23.

23x+23y = 27x+17y;

-4x+6y = 0;

6y = 4x;

x/y = 6/4.

So 3:2.