A 270 metres long train running at speed of 120 kmph crosses anor train running in opposite direction at speed of 80 kmph in 9 seconds. What is length of or train?

$$\eqalign{ & {\text{Relative}}\,{\text{speed}} \cr & = \left( {120 + 80} \right)\,{\text{km/hr}} \cr & = {200 \times \frac{5}{{18}}} \,{\text{m/sec}} \cr & = {\frac{{500}}{9}} \,{\text{m/sec}} \cr & {\text{Let}}\,{\text{the}}\,{\text{length}}\,{\text{of}}\,{\text{the}}\,{\text{other}}\,{\text{train}}\,{\text{be}}\,{\text{x}}\,{\text{metres}}{\text{.}} \cr & {\text{Then,}}\,\frac{{x + 270}}{9} = \frac{{500}}{9} \cr & \Rightarrow x + 270 = 500 \cr & \Rightarrow x = 230 \cr} $$