Question

If a boat goes 7 km upstream in 42 minutes and the speed of the stream is 3 kmph, then the speed of the boat in still water is:

• A. 12 km/hr
• B. 15 km/hr
• C. 14 km/hr
• D. 13 km/hr

Hint:

For upstream motion: \[ \text{Effective speed} = \text{Boat speed} - \text{Stream speed} \] Always convert minutes to hours when working with km/hr units.

Answer 1

The Correct Option is D

Step 1: Convert time to hours. \[ 42 \text{ minutes} = \frac{42}{60} = 0.7 \text{ hours} \] Step 2: Calculate the speed upstream. \[ \text{Speed upstream} = \frac{\text{Distance}}{\text{Time}} = \frac{7}{0.7} = 10 \text{ km/hr} \] Step 3: Use the formula: \[ \text{Speed upstream} = \text{Speed of boat in still water} - \text{Speed of stream} \Rightarrow 10 = B - 3 \Rightarrow B = 13 \text{ km/hr} \]