Discussion
A person can row at 9 kmph and still water. He takes 4 1/2 hours to row from A to B and back. What is the distance between A and B if the speed of the stream is 1 kmph?
Answer: D
With \( b \) the still-water speed and \( r \) the stream's,
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
\[ b + r = 10 \text{ kmph}, \qquad b - r = 8 \text{ kmph} \]
\[ \frac{D}{10} + \frac{D}{8} = 4.5 \]
\[ D\left(\frac{4 + 5}{40}\right) = 4.5 \quad\Longrightarrow\quad \frac{9D}{40} = 4.5 \]
\[ D = 20 \text{ km} \]
20 km appears nowhere among the choices, so the answer is none of these. Checking: 2 hours out and 2½ back, which is the stated 4½ hours.
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