Correct Answer - Option 3 : 900 m

**Given:**

Speed of train A = Speed of train B + 54 km/h

Time taken by train B to cross a man standing in train A = 1 minute

**Formula:**

Speed in km/h to m/s = multiply by 5/18

Distance = Speed × Time

Relative speed while going in the same direction = Difference of the speeds

1 minute = 60 seconds

**Calculation:**

Let speed of train B be x

So, speed of train A = x + 54

The difference in the speed of Train A and Train B = x + 54 – x = 54 km/h

54 km/h = 54 × 5/18 = 15 m/s

We know, relative speed = difference in the speed

Relative speed = 15 m/s

Time = 1 minute = 60 s

Distance = 15 × 60 = 900 m

**∴** **Length of the train B is 900 m**