Aptitude - Problems on Trains - Discussion

Here, mentioning the 2nd train is not needed it is a straight forward question.

The total distance that the trains have to travel to cross each other is "x" and not " x + x/2 ". Where "x" is the length of the longer train. This is where we are going wrong.

Then the length of the longer train would come out to be 300m and the length of the platform will come out to be 300m too.

Which adds up to 600m. And the train will take 45s exactly to cross the platform and the answer.

How come 5/18 and what it mean?

Why he took first train length? Why can't second?

Read the question carefully.

Can any one explain why should we consider second train length as "x/2"?

Correct Answer is 200m.

Two cross Platform Distance would be 400+y.

i.e. If train length is 200 than it should be 400+y.

Let length of first train be 'L' therefore length of 2nd train is 'L/2'.

Now, since trains are moving in opposite direction, there relative speed is,

(48+42) km/h = (L+L/2) /12.

(48+42) x 5/18 m/sec = (3/2 L) /12.

90 x 5/18 x 12 = 3/2 L.

300 = 3/2 L => L = 300 x 2/3 => L = 200 mtrs.

Now with respect to Plateform, Let distance of Platform be 'X'.

Therefore, Total Length = 200 + X and Speed is of first train i.e. 48 Kms/hr & time = 45 secs.

48 kms/hr = 48x5/18 mtrs /sec.

Therefore, 48 x 5/8 = 200 + X/45.

(48x5x45) /18 = 200 + X => 600 = 200 + X => X = 600-200.

X = 400 mtrs.

Answer will be be 400 meter because you guys did not read question properly.

It is clearly mention mean n last line the word "it" is referred for first train.

So now you have length of first train, its time to cover platform of length y meter and its speed 45 km/hr now use simple formula.

s(48km/hr) = (length of train(200)+length of platform(y)) /time to cover/cross platform(45 sec).

So y=400 meter.

I didn't understand any bit of the problem. Please some one do explain in an easy way :).