SOLUTION: A boat travels 9 km upstream and 9 km back. The time for the round trip is 3 hrs. The speed of the stream is 5 ​km/hr. What is the speed of the boat in still​ water?

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Question 1116608: A boat travels 9 km upstream and 9 km back. The time for the round trip is 3 hrs. The speed of the stream is 5 ​km/hr. What is the speed of the boat in still​ water?
Answer by josgarithmetic(39616) About Me  (Show Source):
You can put this solution on YOUR website!
r, boat speed if no stream current 
           SPEED       TIME             DISTANCE

UP          r-5        9%2F%28r-5%29           9

DOWN        r+5        9%2F%28r%2B5%29           9

TOTAL                  3

highlight_green%289%2F%28r-5%29%2B9%2F%28r%2B5%29=3%29
-
3%2F%28r-5%29%2B3%2F%28r%2B5%29=1

3%28r%2B5%29%2B3%28r-5%29=r%5E2-25
6r=r%5E2-25
r%5E2-25-6r=0
r%5E2-6r-25=0
--
discriminant 36%2B4%2A25=136=4%2A34

r=%286%2B2%2Asqrt%2834%29%29%2F2
highlight%28r=3%2Bsqrt%2834%29%29

8.8 km/hour