A man can row 15 km/h downstream and 9 km/h upstream. The speed of the boat in still water is:

Options

A. A)8km/h
B. B)10 km/h
C. C)15 km/h
D. D)12 km/h

Answer

To find the speed of the boat in still water, we can use the formula that relates the speeds downstream and upstream to the speed in still water.

### Step-by-Step Solution:

1. **Identify the given speeds:**
- Speed downstream (S_d) = 15 km/h
- Speed upstream (S_u) = 9 km/h

2. **Use the formula for speed in still water (S_b):**
The speed of the boat in still water can be calculated using the formula:
\[ S_b = \frac{S_d + S_u}{2} \]

3. **Substitute the values into the formula:**
\[ S_b = \frac{15 + 9}{2} \]

4. **Calculate the sum of the speeds:**
\[ S_b = \frac{24}{2} \]

5. **Divide to find the speed in still water:**
\[ S_b = 12 \text{ km/h} \]

### Final Answer:
The speed of the boat in still water is **12 km/h**.

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