SOLUTION: alvin paddled for 3 hours with a 6kmh current to reach a campsite. the return trip against the same current took 9 hours. find the speed of the boat in still water?

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Question 718802: alvin paddled for 3 hours with a 6kmh current to reach a campsite. the return trip against the same current took 9 hours. find the speed of the boat in still water?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let +s+ = the speed in mi/hr of the boat in still water
Let +d+ = the one-way distance he paddles in miles
Going with the current:
(1) +d+=+%28+s+%2B+6+%29%2A3+
Going against the current:
(2) +d+=+%28+s+-+6+%29%2A9+
-------------------
By substitution:
(2) +%28+s+%2B+6+%29%2A3+=+%28+s+-+6+%29%2A9+
(2) +3s+%2B+18+=+9s+-+54+
(2) +6s+=+18+%2B+54+
(2) +6s+=+72+
(2) +s+=+12+
The speed of the boat in still water is 12 mi/hr
check answer:
(1) +d+=+%28+12+%2B+6+%29%2A3+
(1) +d+=+18%2A3+
(1) +d+=+54+
and
(2) +d+=+%28+12+-+6+%29%2A9+
(2) +d+=+6%2A9+
(2) +d+=+54+
OK