A boat rows 18 km downstream and back in 8 hours. If speed doubled, same trip in 3.2 hours. Find speed of boat in still water.

9 km/hr
5 km/hr
6 km/hr
8 km/hr

Correct answer: 6 km/hr

Solution

Let b=boat speed, s=stream speed. Eq1: 36b/(b²-s²)=8 → s²=b²-9b/2. Eq2: 72b/(4b²-s²)=3.2 → 72b/(3b²+9b/2)=3.2 → 72/(3b+4.5)=3.2 → 3b=18 → b=6 km/hr.

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