SOLUTION: A boat makes a 120-mile trip downstream in 3 hours but makes the return trip in 4 hours. What is the rate of the current?

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Question 553486: A boat makes a 120-mile trip downstream in 3 hours but makes the return trip in 4 hours. What is the rate of the current?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let +s+ = the speed of the boat in still water
Let +c+ = the rate of the current
+s+%2B+c+ = the speed of the boat going downstream
+s+-+c+ = the speed of the boat going upstream
------------
given:
Going downstream:
(1) +120+=+%28+s+%2B+c+%29%2A3+
Going upstream:
(2) +120+=+%28+s+-+c+%29%2A4+
--------------------
(1) +3s+%2B+3c+=+120+
(2) +4s+-+4c+=+120+
--------------------
(1) +s+%2B+c+=+40+
(2) +s+-+c+=+30+
Add the equations
+2s+=+70+
+s+=+35+
and
(1) +s+%2B+c+=+40+
(1) +35+%2B+c+=+40+
(1) +c+=+5+
The speed of the current is 5 mi/hr