Answer : 3. "200 mts"
Explanation :

Answer: C) 200 mts Explanation: Let the length of the train be 'L' mts let the speed of the train be 'S' m/s Given it crosses a pole in 10 sec=> L/S = 10 ......(1)Given it takes 20 sec (double of pole) to cross a platform of length 200 mts => (L + 200)/S = 20 => L/S + 200/S = 20 But from (1) L/S = 10 => 200/S = 20 - 10 => S = 20 m/s Then, from (1) => L = 10 x 20 = 200 mts. Hence, the length of the train = 200 mts.