Two trains of equal length are running on parallel lines in the same direction at 46km/hr. and 36km/hr. The faster train passes the slower train in 36 seconds. The length of each train is.
A. 50m
B. 72m
C. 80m
D. 82m

Hint: In this type of question firstly detect that both trains are running in the same direction or opposite direction, then solve it.

Complete step-by-step answer:
Let the length of both trains be x metre.
So the total distance will be \[x+x=2x\] metre
Relative speed \[=46-36\]
          \[=10\,\,km/hr\]
Now convert km/hr into m/sec
As \[1\,km=1000\,m\]
And \[1\,hr=60\times 60\,\sec \]
So, \[10\,km/hr=\dfrac{10\times 1000}{60\times 60}=\dfrac{25}{9}\,m/\sec \]
\[\therefore \dfrac{2x}{36}=\dfrac{25}{9}\]
     \[2x=\dfrac{25\times 36}{9}\]
     \[x=\dfrac{25\times 36}{2\times 9}\]
     \[x=50\,m\]
The correct option is A.

Note: In such types of questions firstly convert units and then solve it accordingly. With this speed the quick train passes a man sitting in the slower train. That is the train needs to run the separation which is equivalent to the length of the train.