Engineering Board Exam Reviewer · Motion Problems · hard
A man rows 12 km downstream and back to his starting point in 2.5 hours. If the speed of the stream is 2 km/h, what is his rowing speed in still water?
- 10 km/h
- 8 km/h
- 12 km/h
- 6 km/h
Answer
A. 10 km/h
Solution
Let v be the speed in still water: 12/v+2+12/v-2=2.5 24v/v2-4=2.5 24v=2.5(v2-4) 5v2-48v-20=0. Factoring, (v-10)(5v+2)=0, so v=10 km/h (the negative root is rejected).
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