The speed of boat in downstream is 15 km/hr and the speed of the current is 2.5 km/hr. The speed of boat against the current is:

Options

A. 8.5 km/hr
B. 9 km/hr
C. 10 km/hr
D. 12.5 km/hr

Answer

To find the speed of the boat against the current, we can use the relationship between the speed of the boat in downstream, the speed of the current, and the speed of the boat against the current.

### Step-by-Step Solution:

1. **Identify the given values:**
- Speed of the boat in downstream (downstream speed) = 15 km/hr
- Speed of the current = 2.5 km/hr

2. **Understand the relationship:**
- The speed of the boat in downstream is the sum of the speed of the boat in still water and the speed of the current.
- The speed of the boat against the current is the speed of the boat in still water minus the speed of the current.

3. **Set up the equations:**
- Let the speed of the boat in still water be \( b \).
- Therefore, in downstream:
\[ b + \text{speed of current} = \text{downstream speed} \]
\[ b + 2.5 = 15 \]

4. **Solve for \( b \):**
\[ b = 15 - 2.5 \]
\[ b = 12.5 \text{ km/hr} \]

5. **Calculate the speed of the boat against the current:**
- Now, using the speed of the boat in still water:
\[ \text{Speed against current} = b - \text{speed of current} \]
\[ \text{Speed against current} = 12.5 - 2.5 \]
\[ \text{Speed against current} = 10 \text{ km/hr} \]

### Final Answer:
The speed of the boat against the current is **10 km/hr**.