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A train with 90 kmph crosses a bridge in 36 s. Another train 100 m shorter crosses the same bridge at 45 kmph. The time taken by the second train to cross the bridge will be

Solution

✅ Correct Option: 2

Understanding what we need to find: When a train crosses a bridge, it travels a distance equal to (length of train + length of bridge).


Convert speeds from kmph to m/s since time is given in seconds:

90 kmph=90×518=25 m/s90 \text{ kmph} = 90 \times \dfrac{5}{18} = 25 \text{ m/s}

45 kmph=45×518=12.5 m/s45 \text{ kmph} = 45 \times \dfrac{5}{18} = 12.5 \text{ m/s}


For the first train, using Distance = Speed × Time:

Length of train+Length of bridge=25×36=900 m\text{Length of train} + \text{Length of bridge} = 25 \times 36 = 900 \text{ m}


Let L=L = length of first train

Then:

  • Length of second train =L−100= L - 100 (given: 100 m shorter)
  • Length of bridge =900−L= 900 - L (from previous step)

For second train to cross the bridge:

Distance to cover=(L−100)+(900−L)=800 m\text{Distance to cover} = (L - 100) + (900 - L) = 800 \text{ m}

Notice: The LL terms cancel out, so we don't need to find individual lengths.


Calculate time for second train:

Time=DistanceSpeed=80012.5=64 seconds\text{Time} = \dfrac{\text{Distance}}{\text{Speed}} = \dfrac{800}{12.5} = 64 \text{ seconds}

Therefore, the second train takes 64 seconds to cross the bridge.

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