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Algebra help

A boat travels 16 miles upstream and 15 miles downstream. The total time for both parts of the trip is 9 hours. The speed of the boat in still water is 9 mph. Find the speed of the current. 

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Ralph L. | Algebra I, II, Visual Basic, Beginning C++ tutorAlgebra I, II, Visual Basic, Beginning C...
4.0 4.0 (1 lesson ratings) (1)
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let x be the speed of the current:
 
16 = (9 - x)t
 
t= 16/(9 - x)
 
substitute t into equation below:
 
15 = (9 + x)(9- t)   ----> -9x2 -x + 450 use quadratic formula ---> [1 ± √[1 - 4*(-9)*450]]/2*(-9)
 
so x ≈ 7.016miles/hour
 
 
Brad M. | Professional Tutor: STEM plus Business, Acc'ting, Investment, EditingProfessional Tutor: STEM plus Business,...
4.9 4.9 (222 lesson ratings) (222)
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Hello again, Erica -- in still water, we could travel 81 miles in 9hr ... current is serious ...
8mph current takes 16hr up ... 7mph current takes 8hr up and 15mi/16mph ~1hr back ...
current is about 7mph  ... Best wishes, ma'am :)

Comments

Hey Brad. What formula did you use? How did you come up with this solition?