Correct Answer - Option 4 : 6 seconds

**Given:**

A 120m long train crosses a 280m long tunnel in 20 seconds.

**Formula Used:**

Time = [ L1 + L2] / Relative speed

Where, L1 = length of train; L2 = length of object that train is crossing

Relative Speed (RS) = Speed of train + speed of platform(= 0)

**Calculation:**

20 seconds = [120 + 280]/RS

⇒ 20 seconds = [120 + 280]/Speed of train [∵ speed of platform = 0]

⇒ Speed of train = 400/20 = 20 m/s

Now, time taken by train to cross the pole = Distance/Speed

Also, length of pole = 0 and speed of pole = 0

⇒ Time taken by train to cross the pole = Length of train / Speed of train

= 120/20 = 6 seconds