Correct Answer - Option 1 : 1 km/h

**Given**

Down stream distance = 21 km

Time taken in downstream = 7 hours

Distance covered in upstream = (1/3) × 21

= 7 km

Time taken in upstream = 7 km

**Formula used**

Speed of stream = (Downstream speed - Upstream speed)/2

Speed = Distance/Time

**Calculation**

Downstream speed = 21/7 = 3 km/h

Upstream speed = [21 × (1/3)]/7 = 7/7

⇒ 1 km/h

Speed of stream = (Downstream speed - Upstream speed)/2

⇒ (3 - 1)/2 = 2/2

⇒ 1 km/h

**∴ Speed of stream is 1 km/h**