A coolie standing on a railway platform observe that a train going in one direction takes 4 second to pass him. Another train of same length going in opposite direction takes 5 seconds to pass him. The time taken (in seconds) by two train to cross each other will be:

Explanation:-

Let length of each train be X m, then $$\eqalign{ & {S_1} = \frac{X}{4}\,{\text{and}} \cr & {S_2} = \frac{X}{5} \cr & {\text{Required time to cross each other,}} \cr & = \left\{ {\frac{{2X}}{{\left[ {\left( {\frac{X}{4}} \right) + \left( {\frac{X}{5}} \right)} \right]}}} \right\} \cr & = \frac{{40}}{9}{\text{seconds}} \cr}$$