Multiple choice

A man standing on a platform finds that a train takes 3 seconds to pass him and another train of the same length, moving in the opposite direction, takes 4 seconds. The time taken by the trains to pass each other will be

  1. 7 seconds

  2. 24/7 seconds

  3. 1 second

  4. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let the length of each train be L. Speed of train 1 = L/3. Speed of train 2 = L/4. When moving in opposite directions, relative speed = L/3 + L/4 = 7L/12. Time to pass each other = (L + L) / (7L/12) = 2L / (7L/12) = 24/7 seconds.

AI explanation

Let the speed of the train be S and the man's speed be zero, making the time to pass the man 3 seconds. When a train of the same length moving in the opposite direction passes the man, the relative speed is S plus the second train's speed, and the time taken is 4 seconds. The length of the first train is 3S, and the length of the second train is also 3S, so its speed is 3S / 4 = 0.75S. When passing each other, their relative speed is S + 0.75S = 1.75S, and the total distance is 6S. The time taken is 6S divided by 1.75S, which equals 24/7 seconds.

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