Multiple choice

Two trains are moving in the opposite directions on parallel tracks at the speeds of $64$ km/hr and $96$ km/hr respectively. The first train passes a telegraph post in $5$ seconds whereas the second train passes the post in $6$ seconds. Find the time taken by the trains to cross each other completely:

  1. $3$ seconds
  2. $\displaystyle 4\frac{4}{5}$ seconds
  3. $\displaystyle 5\frac{3}{5}$ seconds
  4. $6$ seconds
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C Correct answer
Explanation

Let L1 and L2 be lengths. Speed 1 = 64 km/hr = 17.77 m/s, Speed 2 = 96 km/hr = 26.66 m/s. L1 = 17.77 * 5 = 88.88 m. L2 = 26.66 * 6 = 160 m. Relative speed = 160 km/hr = 44.44 m/s. Time = (88.88 + 160) / 44.44 = 5.6 seconds, which is 5 3/5 seconds.

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