Rowing upstream takes a man twice as long as rowing the same distance downstream. Given a still water speed of 42 kmph, find the current speed.
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Answer: C. 14 kmph
Explanation
Rowing upstream takes twice as long, meaning the upstream speed is half of the downstream speed. Letting the current speed be c, upstream speed is forty two minus c and downstream speed is forty two plus c. Solving the proportional speed equation establishes that the current speed equals fourteen kilometers per hour. Time taken to travel a fixed distance is inversely proportional to the effective speed of travel. Downstream speed combines still water boat speed and current speed because the water pushes the vessel forward. Upstream speed subtracts current speed from still water speed because the vessel must fight the flowing water.
Twelve kilometers per hour underestimates the current speed required to satisfy the time ratio given in the problem. Thirteen kilometers per hour fails to balance the inverse time relationship between upstream and downstream journeys. Fifteen kilometers per hour overestimates the current speed needed to produce a two to one time ratio.
- Subject
- Mathematics
- Category
- Mathematics
- Topic
- —
- Difficulty
- medium
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