- 3 km/h
- 3.5 km/h
- 4 km/h
- 4.5 km/h
- 5 km/hr

Option 2 : 3.5 km/h

**Given:**

D = 6 km

Time upstream = 2 hrs

Time downstream = 1.5 hrs

**Concept:**

Speed upstream is boat's speed - stream's speed

Speed downstream is boat's speed + stream's speed

**Formula used:**

Speed = distance/time

speed in still water = (speed upstream + speed downstream)/2

**Calculation:**

Speed downstream = 6/1.5 = 4 km/h

Speed upstream = 6/2 = 3 km/h

Speed in still water = (4 + 3)/2 = 3.5 km/h

**∴ The speed in still water is 3.5 km/h**

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