SOLUTION: Don Williams uses his small motorboat to go 4 miles upstream to his favorite fishing spot. Against the current, the trip takes 2/5 hours. With the current the trip takes 1/5. How f

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Question 720809: Don Williams uses his small motorboat to go 4 miles upstream to his favorite fishing spot. Against the current, the trip takes 2/5 hours. With the current the trip takes 1/5. How fast can the boat travel in still water. What is the speed of the current?? thanks
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 speed of ther current
+s+-+c+ = the speed of the boat going against the current
+s+%2B+c+ = the speed of the boat going with the current
-----------------
Going upstream:
(1) +4+=+%28+s+-+c+%29%2A%282%2F5%29+
Going downstream:
(2) +4+=+%28+s+%2B+c+%29%2A%281%2F5%29+
----------------------
(1) +20+=+2s+-+2c+
(1) +10+=+s+-+c+
and
(2) +20+=+s+%2B+c+
----------------
Add (1) and (2)
+30+=+2s+
+s+=+15+
and
(1) +10+=+s+-+c+
(1) +10+=+15+-+c+
(1) +c+=+15+-+10+
(1) +c+=+5+
The speed of the boat in still water is 15 mi/hr
The speed of ther current is 5 mi/hr
check:
(2) +4+=+%28+s+%2B+c+%29%2A%281%2F5%29+
(2) +4+=+%28+15+%2B+5+%29%2A%281%2F5%29+
(2) +4+=+20%2F5+
(2) +4+=+4+
OK
(1) +4+=+%28+s+-+c+%29%2A%282%2F5%29+
(1) +4+=+%28+15+-+5+%29%2A%282%2F5%29+
(1) +4+=+20%2F5+
(10 +4+=+4+
OK