The speed of a boat in still water is 12 kmph . If the boat covers 36 km upstream in 4 hours , what is the speed of stream ?

Options

A. 3 km/hr
B. 4 km/hr
C. 5 km/hr
D. 2 km/hr

Explanation

To solve the problem step by step, we can follow these instructions:

### Step 1: Understand the given information
- The speed of the boat in still water = 12 km/h
- Distance covered upstream = 36 km
- Time taken to cover this distance = 4 hours

### Step 2: Define the speed of the stream
Let the speed of the stream be \( x \) km/h.

### Step 3: Calculate the effective speed of the boat upstream
When the boat is moving upstream, its effective speed is reduced by the speed of the stream. Therefore, the effective speed of the boat upstream is:
\[ \text{Effective speed upstream} = \text{Speed of boat} - \text{Speed of stream} = 12 - x \text{ km/h} \]

### Step 4: Use the formula for speed, time, and distance
We know that:
\[ \text{Speed} = \frac{\text{Distance}}{\text{Time}} \]
From the problem, we can set up the equation:
\[ 12 - x = \frac{36 \text{ km}}{4 \text{ hours}} \]

### Step 5: Simplify the equation
Calculate the right side:
\[ \frac{36}{4} = 9 \text{ km/h} \]
So, we have:
\[ 12 - x = 9 \]

### Step 6: Solve for \( x \)
Rearranging the equation gives:
\[ x = 12 - 9 \]
\[ x = 3 \text{ km/h} \]

### Conclusion
The speed of the stream is **3 km/h**.

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