Two trains 105 meters and 90 meters long, run at the speeds of 45 kmph and 72 kmph respectively, in opposite directions on parallel tracks. The time which they take to cross each other, is:

Length of the 1^st train = 105 m
Length of the 2^nd train = 90 m
Relative speed of the trains,
= 45 + 72 = 117 kmph
= $$\frac{{117 \times 5}}{{18}}$$ = 32.5 m/sec
Time taken to cross each other,
= $$\frac{{{\text{Length}}\,{\text{of}}\,{1^{{\text{st}}}}\,{\text{train}} + {\text{length of}}\,{2^{{\text{nd}}}}\,{\text{train}}}}{{{\text{relative }}\,{\text{speed }}\,{\text{of}}\,{\text{the }}\,{\text{trains}}}}$$
∴ Time taken = $$\frac{{195}}{{32.5}}$$  = 6 secs.