SOLUTION: A boat traveled 210 miles downstram and back. the trip downstram took 10 hours. the trip back took 70 hours. what is the speed of the boat in still water? what is the speed of the

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Question 559322: A boat traveled 210 miles downstram and back. the trip downstram took 10 hours. the trip back took 70 hours. what is the speed of the boat in still water? what is the speed of the current?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let s = speed of boat in still water
Let c = speed of current
+s+%2B+c+ = speed going downstream
+s+-+c+ = speed going upstream
------------------------------
Going downstream:
(1) +210+=+%28+s+%2B+c+%29%2A10+
Going upstream:
(2) +210+=+%28+s+-+c+%29%2A70+
-------------------
(1) +10s+%2B+10c+=+210+
(2) +70s+-+70c+=+210+
Multiply both sides of (1) by 7
and add the equations
(1) +70s+%2B+70c+=+1470+
(2) +70s+-+70c+=+210+
+140s+=+1680+
+s+=+12+
and
(1) +10s+%2B+10c+=+210+
(1) +s+%2B+c+=+21+
(1) +12+%2B+c+=+21+
(1) +c+=+9+
12 mi/hr = speed of boat in still water
9 mi/hr = speed of current
check answer:
(2) +70s+-+70c+=+210+
(2) +s+-+c+=+3+
(2) +12+-+9+=+3+
(2) +3+=+3+
OK