SOLUTION: Gone fishing. Debbie traveled by boat 5 miles upstream to fish in her favorite spot. Because of the 4-mph current, it took her 20 minutes longer to get there than to return. How

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Question 204108This question is from textbook
: Gone fishing. Debbie traveled by boat 5 miles upstream to
fish in her favorite spot. Because of the 4-mph current, it
took her 20 minutes longer to get there than to return. How
fast will her boat go in still water?

Thank you for your help This question is from textbook

Answer by ankor@dixie-net.com(22740) About Me  (Show Source):
You can put this solution on YOUR website!
Debbie traveled by boat 5 miles upstream to fish in her favorite spot.
Because of the 4 mph current, it took her 20 minutes longer to get there
than to return.
How fast will her boat go in still water?
:
Let s = boat speed in still water
then
(s-4) = speed upstream
and
(s+4) = speed down stream
:
Convert 20 min to hrs: 20/60 = 1%2F3hr
:
Write a time equation: time = distance%2Fspeed
Upstream time = downstream time + 20 minutes (1/3 hr)
5%2F%28%28s-4%29%29 = 5%2F%28%28s%2B4%29%29 + 1%2F3
:
To clear out these denominators, multiply equation by 3(s+4)(s-4)
3(s+4)(s-4)*5%2F%28%28s-4%29%29 = 3(s+4)(s-4)*5%2F%28%28s%2B4%29%29 + 1%2F3*3(s+4)(s-4)
Results in
:
15(s+4) = 15(s-4) + (s+4)(s-4)
:
15s + 60 = 15s - 60 + s^2 - 16
:
0 = 15s - 15s - 60 - 60 + s^2 - 16
:
0 = s^2 - 136
:
s^2 = 136
s = sqrt%28136%29
s = 11.66 mph speed in still water
:
:
Check solution on calc: (speed up = 7.66 and speed down = 15.66)
5/7.66 - 5/15.66 = .3333 which is 1/3 of an hr