Institution: PROBASHI KALLYAN BANK Exam: প্রবাসী কল্যাণ ব্যাংক-সিনিয়র অফিসার-09-10-2021
Topic: Arithmetic Mathematics (পাটিগণিত), Subject: Mathematics (সাধারণ গণিত)

Best Answer Explained in Below-:

To find the speed of the stream, we first need to determine the speed of the boat in still water and the speed of the stream using the given information.

Let the speed of the boat in still water be \( b \) km/hr and the speed of the stream be \( s \) km/hr.

When the boat is sailing against the stream, its effective speed becomes \( b - s \). The time taken to travel 24 km against the stream is 6 hours. Using the formula \( \text{Speed} = \frac{\text{Distance}}{\text{Time}} \), we get:
\[
b - s = \frac{24}{6} = 4 \, \text{km/hr} \quad \text{(1)}
\]

When the boat is sailing with the stream, its effective speed is \( b + s \). The time taken to travel 24 km with the stream is 4 hours, so:
\[
b + s = \frac{24}{4} = 6 \, \text{km/hr} \quad \text{(2)}
\]

Now we have a system of equations:
1. \( b - s = 4 \) (1)
2. \( b + s = 6 \) (2)

We can solve these equations by adding them:
\[
(b - s) + (b + s) = 4 + 6 \\
2b = 10 \\
b = 5 \, \text{km/hr}
\]

Substituting \( b = 5 \) back into equation (1):
\[
5 - s = 4 \\
s = 1 \, \text{km/hr}
\]

Thus, the speed of the stream is 1 km/hr, which corresponds to option C.