SOLUTION: A fishing boat traveled 30 miles down a river and then returned. The total time for the round trip was 4 hours, and the rate of the river's current was 4mph. Find the rate of the b

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Question 447028: A fishing boat traveled 30 miles down a river and then returned. The total time for the round trip was 4 hours, and the rate of the river's current was 4mph. Find the rate of the boat in still water.
I thought this might be the formula: 30/4+x + 30/4-x =4 but it's not working out. Please help!
Found 2 solutions by Alan3354, htmentor:
Answer by Alan3354(69443) (Show Source):
You can put this solution on YOUR website!
A fishing boat traveled 30 miles down a river and then returned. The total time for the round trip was 4 hours, and the rate of the river's current was 4mph. Find the rate of the boat in still water.
I thought this might be the formula: 30/4+x + 30/4-x =4 but it's not working out.
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30/4+x + 30/4-x = 4 The boat's speed has to be greater than 4, or it would never get back. So it's x-4, not 4-x
30/(4+x) + 30/(x-4) = 4
30(x-4) + 30(x+4) = 4(x-4)(x+4) = 4x^2 - 64
60x = 4x^2 - 64
x^2 - 15x - 16 = 0
(x-16)*(x+1) = 0
x = -1 Ignore
x = 16 mph

Answer by htmentor(1343) (Show Source):
You can put this solution on YOUR website!
Your reasoning is correct but the formula is slightly wrong.
If the rate in still water is x, then the rate with the current is x+4, and the
rate against the current is x-4 (not 4-x).
Otherwise, the rate in still water would have to be LESS than the rate of the
current, which means that the boat would never get there!
Try it this way
30/x+4 + 30/x-4 = 4
and see if you get the answer x = 16 mph