Question : Two rifles are fired from the same place at a difference of 11 minutes 45 seconds. But a man who is coming towards the same place on a train hears the second sound after 11 minutes. Find the speed of the train (Assuming the speed of sound = 330 m/s).
Option 1: 72 km/h
Option 2: 36 km/h
Option 3: 81 km/h
Option 4: 108 km/h
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Correct Answer: 81 km/h
Solution : Given: Time interval between two rifle fires = 11 minutes 45 seconds = 705 seconds Time taken for the man to hear the second sound = 11 minutes = 660 seconds Formula used: Distance travelled by the man on a train in 11 minutes = distance travelled by sound in (11 min 45 sec - 11 min) = 45 sec. Let the speed of the train be $x$ kmph. Now, $x\times \frac{11}{60}=\frac{45}{3600}\times 330\times \frac{18}{5}$ $x$ = 81 km/h The speed of the train is 81 km/h. Hence, the correct answer is 81 km/h.
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