SOLUTION: 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, the

Algebra ->  Customizable Word Problem Solvers  -> Misc -> SOLUTION: 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, the      Log On

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Question 201087: 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 josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let c = speed of current
Let b = speed of boat in still water
Let t = time for both trips in hrs
given:
b+=+15 mi/hr
12+=+%2815+%2B+c%29%2At
9+=+%2815+-+c%29%2At
-------------------
12+=+15t+%2B+ct
9+=+15t+-+ct
Add the equations
21+=+30t
t+=+7%2F10 hrs
And, since
12+=+%2815+%2B+c%29%2At
12+=+%2815+%2B+c%29%2A%287%2F10%29
120+=+105+%2B+7c
7c+=+15
c+=+2.143 mi/hr answer
check:
12+=+15t+%2B+ct
12+=+15%2A.7+%2B+2.143%2A.7
12+=+10.5+%2B+1.5
12+=+12
and
9+=+%2815+-+c%29%2At
9+=+%2815+-+2.143%29%2A.7
9+=+12.857%2A.7
9+=+8.9999 close enough