Correct Answer - Option 3 : 6 km/hr

**Given:**

Upstream speed = 28 km/hr

Downstream speed = 40 km/hr

**Concept Used:**

Upstream speed = (x – y) km/hr

Downstream speed = (x + y) km/hr

where,

x is the speed of a boat in still water.

y is the speed of a stream.

**Calculation:**

⇒ x + y = 40 ----(1)

⇒ x – y = 28 ----(2)

Adding both the equation we get,

⇒ 2x = 68

⇒ x = 34

Putting the above value in equation 1,

⇒ 34 + y = 40

⇒ y = 40 – 34

⇒ y = 6

**∴ The speed of stream is 6 km/hr.**