CMA Foundation · Fundamentals of Business Mathematics and Statistics · Time and Distance
A boat takes twice as long to go a certain distance upstream as to come back downstream. If the stream speed is 4 km/h, what is the boat's speed in still water?
The still-water speed is 12 km/h. Since upstream takes twice the time, downstream speed is double the upstream speed: b + 4 = 2(b - 4), giving b = 12. Downstream is 16 km/h and upstream 8 km/h.
- A12 km/hCorrect
- B8 km/h
- C16 km/h
- D6 km/h
Explanation
Time upstream is twice time downstream, so downstream speed is twice upstream speed: b+4 = 2(b-4). So b+4 = 2b-8 and b = 12 km/h. Check: downstream 16, upstream 8, ratio 2:1. The value 8 km/h would give upstream speed 4 and downstream 12, ratio 3:1.
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