Relative speed when two objects move towards each other = Speed of first object + Speed of second object Time taken to cross each other = Total distance ∕ Relative speed Calculation: Relative speed =50km∕hr+70km∕hr ⇒ Relative speed =120km∕hr Convert speed to m∕s : ⇒120km∕hr=120×(1000∕3600)m∕s ⇒120km∕hr=33.33m∕s Total distance = Length of first train + Length of second train ⇒ Total distance =180m+180m ⇒ Total distance =360m Time taken to cross each other = Total distance ∕ Relative speed ⇒ Time taken =360m∕33.33m∕s ⇒ Time taken =10.8 seconds