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

Asked in JEE Main 13th April 1st Shift 2023 · Distance and average speed

Answer: (3) 1800 m

Step-by-step solution

Speeds: 108 km/h = 30 m/s, 72 km/h = 20 m/s.

To cross the tunnel a train covers the tunnel length plus its own length.

t_A = (L + l)/(30), t_B = (L + 4l)/(20).

With L = 60l: t_A = (61l)/(30), t_B = (64l)/(20) = (16l)/5.

t_B - t_A = (96l - 61l)/(30) = (35l)/(30) = 35 ⇒ l = 30 m.

L = 60l = 1800 m.

Why the other options are wrong

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