Question 197838
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Going downstream, the speed of the current adds to the speed of the boat in still water, so if <b><i>r</i></b> represents the unknown speed of the current, the speed downstream is *[tex \LARGE 15 + r].  Likewise, going upstream, the speed of the current subtracts from the speed in still water, so:  *[tex \LARGE 15 - r]


Using *[tex \LARGE d = rt \ \ \Rightarrow\ \ t = \frac{d}{r}] the downstream trip is described by:


*[tex \LARGE \ \ \ \ \ \ \ \ \ \ t = \frac{12}{15 + r}]


And the upstream trip:


*[tex \LARGE \ \ \ \ \ \ \ \ \ \ t = \frac{9}{15 - r}]


Since the problem says both trips take the same amount of time, set the two fractions equal to each other and solve the proportion by cross-multiplying.


John
*[tex \LARGE e^{i\pi} + 1 = 0]
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