John can run his boat in the still water at a speed of 15 mi

John can run his boat in the still water at a speed of 15 miles per hour. If he can go 12 miles downstream in the same time as it takes him to go 9 miles upstream, then what is the speed of the water currents?

Solution

Let speed of the current be \"c\".
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Downstream DATA:
distance = 12 mi. ; Rate = 15+c mph ; time=d/r=12/(15+c) hrs.
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Upstream DATA:
distance =9mi ; rate = 15-c ; time = d/r= 9/(15-c)
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EQUATION:
time down = time up
12/(15+c) = 9/(15-c)
12(15-c)=9(15+c)
180-12c=135+9c
45=21c
current speed =15/7= 2.15 mph