Two trains A and B of length l and 4 l are travelling into a tunnel of length L in parallel tracks from opposite directions with velocities 108 km h - 1 and 72 km h - 1 , respectively. If train A take 35 s less time than train B to cross the tunnel then, length L of tunnel is: (Given L = 60 l )

Step-by-step solution

The time taken for A Δ t A = l + L V A The time taken for B Δ t B = 4 l + L V B It is given that B takes more time than A . So, ⇒ 4 l + L 20 - l + L 30 = 35 ⇒ 64 l 20 - 61 l 30 = 35 ⇒ l = 35 × 600 700 = 30   m Therefore, L = 1800   m .