Multiple choice

A boy standing on a railway platform observes that a train going in one direction takes 7 seconds to pass him. Another train of same length going in the opposite direction as compare to the previous train takes 9 seconds to pass him. Find the time taken (in seconds) by trains to cross each other.

  1. $\frac{69}{8}$
  2. $\frac{63}{8}$
  3. $\frac{67}{8}$
  4. $\frac{65}{8}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let train length = L and speeds = v1, v2. L/v1 = 7, L/v2 = 9. When crossing: 2L/(v1+v2) = 2/(1/7+1/9) = 2/(16/63) = 63/8 seconds.

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