A train takes 10 seconds to cross a pole and 20 seconds to cross a platform of length 200m. What is the length of the train?

Question: A train takes 10 seconds to cross a pole and 20 seconds to cross a platform of length 200m. What is the length of the train?

Solution:
Let the length of the train be L meters.
When a train crosses a pole, it covers its own length (L).
So, Speed of the train = L / 10
When the train crosses a platform, it covers its own length plus the platform length (L + 200).
So, Speed of the train = (L + 200) / 20

Since the speed is constant:
L/10 = (L + 200)/20
⇒ 20L = 10(L + 200)
⇒ 20L = 10L + 2000
⇒ 20L - 10L = 2000
⇒ 10L = 2000
⇒ L = 2000/10
∴ L = 200

The length of the train is 200m.