Discussion
A 360 m long train A passes a pole in 27 seconds, and passes completely another train B, running in opposite direction at 60 km/h, in 19 seconds. In how many seconds will train B cross completely a bridge of length 540 m?
Answer: D
Train B crosses the bridge in 45 seconds. Train A passes a pole in 27 seconds, so its speed is 360/27 = 40/3 m/s = 48 km/h. Train B runs the opposite way at 60 km/h, so the relative speed is 108 km/h, which is 30 m/s. They cross in 19 seconds, so the sum of their lengths is 30 x 19 = 570 m, and B is 570 - 360 = 210 m long.
\[ \text{Speed of B} = 60 \times \frac{5}{18} \]
\[ = \frac{50}{3} \text{ m/s} \]
\[ \text{Time} = \frac{210 + 540}{50/3} = 45 \text{ s} \]
The options 36, 40 and 42 seconds do not equal 750 m divided by the speed of 50/3 m/s, so the fourth option is correct.
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