SOLUTION: 36. Upstream, downstream. Junior’s boat will go 15 miles per
hour in still water. If he can go 12 miles downstream in the
same amount of time as it takes to go 9 miles upstream,
Algebra.Com
Question 201646: 36. Upstream, downstream. Junior’s boat will go 15 miles per
hour in still water. If he can go 12 miles downstream in the
same amount of time as it takes to go 9 miles upstream,
then what is the speed of the current?
Answer by solver91311(24713) (Show Source): You can put this solution on YOUR website!
Use distance equals rate times time, or 
, or, for this problem, you would want to express time as a relationship between distance and rate, namely 
. The speed of the current subtracts from the speed of the boat when going upstream and adds to the speed of the boat when going downstream, so the upstream trip can be described as:
})
And the downstream trip
})
Since we are told it is the same amount of time, we can equate the two right-hand sides:
}\ =\ \frac{12}{(15 + r_c)})
Cross-multiply:
 = 12(15 - r_c))
Solve for 
John
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