A train crosses a stationary tree in 13 seconds and a 150-meter-long bridge in 18 seconds. If a dog is walking in the same direction as the train at a speed of 4 m/s, how much time will the train take to completely pass the dog?

Let the length of the train = ‘x’ metres Then, distance covered by the train in 13 seconds = ‘x’ metres Distance covered by the train in 18 seconds = (x + 150) metres So, distance covered by the train in 5 seconds i.e. (18 – 13) seconds = (x + 150 – x) metres So, speed of the train = (150/5) = 30 m/s Length of the train = 30 × 13 = 390 metres Relative speed of the train with respect to the dog who is walking in opposite direction = 30 – 4 = 26 m/s So, time taken by the train to cross the dog = 390 ÷ 26 = 15 seconds.