Train 'A' crosses a pole in 35 seconds and takes 40 seconds to cross a platform that is 120 meters long. Train 'B' is half the length of Train 'A' and takes 30 seconds to cross a 210-meter-long platform. How much time will it take for Train 'A' and Train 'B' to cross each other when they are travelling in opposite directions?

Let the length of train 'A' be 'd' metres. So, distance covered by train 'A' in 35 seconds = 'd' metres And distance covered by train 'A' in 40 seconds = (d + 120) metres So, distance covered by train 'A' in (40 - 35) i.e., 5 seconds = d + 120 - d = 120 metres So, speed of train 'A' = (120/5) = 24 m/s So, length of train 'A' = 24 X 35 = 840 metres Length of train 'B' = (840/2) = 420 metres So, speed of train 'B' = (420 + 210) ÷ 30 = 21 m/s So, time taken by trains 'A' and 'B' to cross each other = (840 + 420) ÷ (24 + 21) = 1260 ÷ 45 = 28 seconds