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Two trains cross each other in 1414 seconds when running in opposite directions along parallel tracks. The faster train is 160160 m long and crosses a lamp post in 1212 seconds. If the speed of the other train is 66 km/hr less than the faster one, its length, in m, is

Solution

✅ Correct Option: 1

Let's solve this step-by-step, breaking down this relative speed problem into manageable parts.

The faster train crosses a lamp post in 12 seconds. When a train crosses a lamp post, it travels a distance equal to its own length.

Speed of faster train = Length of trainTime taken=160 m12 s=403 m/s\frac{\text{Length of train}}{\text{Time taken}} = \frac{160 \text{ m}}{12 \text{ s}} = \frac{40}{3} \text{ m/s}


We're told the slower train's speed is 6 km/hr less than the faster train. Since we're working in m/s, let's convert this difference:

To convert km/hr to m/s, we multiply by 518\frac{5}{18}

This is because: 1 km/hr=1000 m3600 s=518 m/s1 \text{ km/hr} = \frac{1000 \text{ m}}{3600 \text{ s}} = \frac{5}{18} \text{ m/s}

Speed difference = 6×518=3018=53 m/s6 \times \frac{5}{18} = \frac{30}{18} = \frac{5}{3} \text{ m/s}


Speed of slower train = 403−53=353 m/s\frac{40}{3} - \frac{5}{3} = \frac{35}{3} \text{ m/s}


When two objects move in opposite directions, their relative speed is the sum of their individual speeds.

Relative speed = 403+353=753=25 m/s\frac{40}{3} + \frac{35}{3} = \frac{75}{3} = 25 \text{ m/s}


When two trains cross each other, the total distance covered equals the sum of their lengths.

Using the formula: Distance = Speed × Time

Total distance = Relative speed × Time = 25×14=350 m25 \times 14 = 350 \text{ m}


Sum of lengths of both trains = 350 m

Length of second train = Total length - Length of first train

Length of second train = 350−160=190 m350 - 160 = 190 \text{ m}


We remember this approach for similar problems: We find individual speeds using given information, calculate relative speed by adding for opposite directions or subtracting for same direction, use relative speed × time = sum of lengths, and solve for the unknown length.

Answer: 190 m

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