A person rows a boat 10 kms along the stream in 30 minutes and returns to the starting point in 40 minutes. The speed of the stream is:

2.5 km/h

The correct answer is Option (2) → 2.5 km/h

Given:

• Distance along the stream = 10 km
• Time downstream = 30 min = 0.5 h
• Time upstream = 40 min = 2/3 h

Let:

• Speed of the boat in still water = b km/h
• Speed of the stream = s km/h

Step 1: Write downstream and upstream speeds

• Downstream speed: $b + s = \frac{\text{distance}}{\text{time}} = \frac{10}{0.5} = 20$
• Upstream speed: $b - s = \frac{10}{2/3} = \frac{10}{0.6667} \approx 15$

Step 2: Solve for s

$b + s = 20 \quad \text{and} \quad b - s = 15$

Add both equations:

$2b = 35 \Rightarrow b = 17.5 \text{ km/h}$

Then:

$s = 20 - 17.5 = 2.5 \text{ km/h}$