Option 2 : 76 km/h

**Given:**

Speed of boy = 8 km/h

Length of train = 280 m

Time taken to cross = 12 seconds

Concept used:

The speed of an object with respect to another is called relative speed.

Suppose two bodies are moving with speeds of x km/h and y km/h respectively, then the relative speed

If both the bodies are moving in opposite directions = (x + y) km/h

If both the bodies are moving in same directions = (x - y) km/h

Formula used:

Speed = Distance/Time

1 km/h = (5/18) m/s

**Calculation:**

Let the speed of the train be x km/h

Relative speed of train with respect to the boy = (x + 8) km/h

Length of train = 280 m = 0.280 km

Time taken by train to cross the man = [0.280/(x + 8)] hours

According to the question

[0.280/(x + 8)] = 12/3600

⇒ [0.280/(x + 8)] = 1/300

⇒ (0.280 × 300) = (x + 8)

⇒ 84 = x + 8

⇒ x = 76 km/h

**∴ Speed of train is 76 km/h.**