Multiple choice

Two trains are running on parallel tracks in the same direction. The length of the two trains is an integral multiple of the number of the bogies. If the difference of their speeds is increased by 20%; and the total number of the bogies of both the trains is decreased by 10%. Find the percentage change in the time of crossing of the slower train by the faster train.

  1. 30%

  2. 25%

  3. 12%

  4. 20%

  5. 22.5%
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Crossing time = total length/relative speed. When relative speed increases by 20% (factor 1.2) and total length decreases by 10% (factor 0.9), new time = (0.9L)/(1.2Δv) = 0.75 × (L/Δv) = 0.75T. This is a 25% decrease.

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