Discussion
A man takes 3 hours 45 minutes to row a boat 15 km downstream of a river and 2 hours 30 minutes to cover a distance of 5 km upstream. Find the speed of the current.
Answer: A
With \( b \) the still-water speed and \( r \) the stream's,
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
\[ 3 \text{ h } 45 \text{ min} = 3.75 \text{ h}, \qquad 2 \text{ h } 30 \text{ min} = 2.5 \text{ h} \]
\[ b + r = \frac{15}{3.75} = 4 \text{ km/hr}, \qquad b - r = \frac{5}{2.5} = 2 \text{ km/hr} \]
\[ r = \frac{4 - 2}{2} = 1 \text{ km/hr} \]
The boat itself manages 3 km/hr. Converting the mixed times to decimal hours before dividing keeps the arithmetic clean.
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