Correct Answer - Option 3 : 95 seconds

**Given,**

Length of train = 400 metres

Time is taken to cross the boundary = 2 minutes

Speed = 72 km/hr

**Formula used,**

Speed of train = (length of train + length of boundary)/time taken to cross the boundary

**Calculation**

Length of train, a = 400 metres

Time taken to cross the boundary = 2 minutes

= 120 seconds

Speed = 72 km/hr

= 72 × (5/18) m/sec

= 20 m/sec

Let length of boundary is b metre.

Speed of train = (a + b)/time taken to cross the boundary

⇒ 20 = (400 + b)/120

⇒ 20 × 120 = 400 + b

⇒ 2400 – 400 = b

Length of boundary, b = 2000 metres

If the boundary would be 500 metres shorter, then its length would be 1500 metres.

New length of boundary = 1500 metres

Speed = 20 m/sec

Time is taken to cross-boundary = (Length of train + length of boundary)/Speed of train

⇒ Time is taken to cross-boundary = (1500 + 400)/20 = 95 seconds

**∴Time is taken to cross the boundary is 95 seconds**