SOLUTION: if it takes 3 hours to travel 15 miles downstream and 5 hours to travel upstream, how fast is the current if the speed of the boat remains constant?

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Question 590923: if it takes 3 hours to travel 15 miles downstream and 5 hours to travel upstream, how fast is the current if the speed of the boat remains constant?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let +s+ = the speed of the boat
Let +c+ = the speed of the current
+s+%2B+c+ = the speed going downstream
+s+-+c+ = the speed going upstream
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Going downstream:
+s+%2B+c+=+15%2F3+
+s+%2B+c+=+5+ mi/hr
Going upstream:
+s+-+c+=+15%2F5+
+s+-+c+=+3+ mi/hr
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Add these equations:
+s+%2B+c+=+5+
+s+-+c+=+3+
+2s+=+8+
+s+=+4+
The speed of the boat is 4 mi/hr
check answer:
+s+%2B+c+=+5+
+4+%2B+c+=+5+
+c+=+1+ mi/hr
Going downstream:
r*t = d
+%28+4+%2B+1%29%2A3+=+15+
Going upstream:
+%28+4+-+1%29%2A5+=+15+
OK