The upstream speed of the boat is 40 km/hr and the speed of the boat in still water is 55 km/hr. What is the downstream speed of the boat ?
A. 75 km/hr
B. 70 km/hr
C. 60 km/hr
D. 65 km/hr

Options

A. C
B. B
C. A
D. D

Answer

To find the downstream speed of the boat, we can follow these steps:

### Step 1: Define the Variables
Let:
- \( x \) = speed of the boat in still water (in km/hr)
- \( y \) = speed of the stream (in km/hr)

### Step 2: Use the Given Information
From the problem, we know:
- The speed of the boat in still water \( x = 55 \) km/hr
- The upstream speed (speed against the current) = \( x - y = 40 \) km/hr

### Step 3: Set Up the Equation
Using the upstream speed information:
\[ x - y = 40 \]
Substituting the value of \( x \):
\[ 55 - y = 40 \]

### Step 4: Solve for \( y \)
Rearranging the equation to find \( y \):
\[ y = 55 - 40 \]
\[ y = 15 \text{ km/hr} \]

### Step 5: Calculate the Downstream Speed
The downstream speed (speed with the current) is given by:
\[ \text{Downstream speed} = x + y \]
Substituting the values of \( x \) and \( y \):
\[ \text{Downstream speed} = 55 + 15 \]
\[ \text{Downstream speed} = 70 \text{ km/hr} \]

### Step 6: Conclusion
Thus, the downstream speed of the boat is **70 km/hr**.