Question

The speed of a boat in still water is 4 km/hr and the speed of current is 2 km/hr. If the time taken to reach a certain distance upstream is 9 hours, find the time it will take to go the same distance downstream.

• A. 2 hrs
• B. 2.5 hrs
• C. 3.5 hrs
• D. 3 hrs

Hint:

For upstream-downstream problems, use: \[ \text{Speed}_{\text{up}} = \text{Boat Speed} - \text{Current Speed} \] \[ \text{Speed}_{\text{down}} = \text{Boat Speed} + \text{Current Speed} \]

Answer 1

The Correct Option is D

Speed upstream: \[ 4 - 2 = 2 \text{ km/hr} \] Let the distance be \( d \), so: \[ \frac{d}{2} = 9 \Rightarrow d = 18 \text{ km} \] Speed downstream: \[ 4 + 2 = 6 \text{ km/hr} \] Time downstream: \[ \frac{18}{6} = 3 \text{ hrs} \] Thus, the correct answer is 3 hrs.