Discussion
A boat covers a certain distance downstream in 1 hour and returns in 1 hour 30 minutes. If the stream flows at 3 km/hr, what is the speed of the boat in still water ?
Answer: C
Correct answer: 15 km/hr
The distance is the same each way, so speed multiplied by time must match:
\[(b + 3) \times 1 = (b - 3) \times 1.5\]
\[b + 3 = 1.5b - 4.5 \quad\Longrightarrow\quad 7.5 = 0.5b \quad\Longrightarrow\quad b = 15 \text{ km/hr}\]
The distance is then \(18 \times 1 = 18\) km, and the return at 12 km/hr indeed takes 1.5 hours.
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