Question

A boat can travel with a speed of 13km/h in still water. If the speed of the stream is 4km/h. Find the time taken by the boat to go 81 km upstream?

• A. 12.4 hr
• B. 540 mins
• C. 9 mins
• D. 91 mins

Hint:

Always check the units in the options carefully. 9 hours is equivalent to 540 minutes ($9 \times 60$), which makes Option (B) correct over Option (C) (which says 9 *minutes*).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
When a boat travels upstream (against the water current), the effective speed of the boat is reduced by the speed of the stream.
Key Formula or Approach:
The formulas are: \[ \text{Upstream Speed } (v_{up}) = \text{Speed in still water } (v) - \text{Speed of stream } (u) \] \[ \text{Time taken } (t) = \frac{\text{Distance } (d)}{v_{up}} \]

Step 2: Detailed Explanation:
Given values from the problem:
- Speed of the boat ($v$) = $13\text{ km/h}$
- Speed of the stream ($u$) = $4\text{ km/h}$
- Distance ($d$) = $81\text{ km}$
First, calculate the upstream speed: \[ v_{up} = 13 - 4 = 9\text{ km/h} \] Second, calculate the time taken: \[ t = \frac{81}{9} = 9\text{ hours} \] Convert the time into minutes: \[ t = 9 \times 60 = 540\text{ minutes} \]

Step 3: Final Answer:
The time taken is 540 minutes, matching Option (B).