Boats & Streams · Question 12 · Hard
Boats & Streams question 12
A boat goes 60 km downstream and returns to the starting point in a total of 8 hours. If the stream flows at 4 km/h, what is the speed of the boat in still water?
- 20 km/h
- 12 km/h
- 15 km/h
- 16 km/h
Answer and explanation
Answer: D. 16 km/h
Explanation: Try b = 16: downstream 20 km/h takes 3 h, upstream 12 km/h takes 5 h, total 8 h. Formally 60/(b + 4) + 60/(b − 4) = 8 simplifies to b² − 15b − 16 = 0, i.e. (b − 16)(b + 1) = 0, so b = 16.
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