SOLUTION: Trina's boat travels 20 km/h in still water. A river has a current that flows at a rate of 5 km/h. Trina travels in her boat downstream for 1 km and then returns to her starting po

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Question 1127041: Trina's boat travels 20 km/h in still water. A river has a current that flows at a rate of 5 km/h. Trina travels in her boat downstream for 1 km and then returns to her starting point. Her average rate, in km, is....?
I keep on getting 25km/h as the answer. I know it's wrong, but I don't know what I'm doing wrong. Thanks for helping.
Found 2 solutions by josgarithmetic, ikleyn:
Answer by josgarithmetic(39617) About Me  (Show Source):
You can put this solution on YOUR website!
downstream, %2820%2B5%29%2Ax=1
x=1%2F25

upstream, %2820-5%29%2Ay=1
y=1%2F15

average speed for the whole round-trip,
%281%2B1%29%2F%281%2F25%2B1%2F15%29

-

2%2F%281%2F25%2B1%2F15%29

%282%2F%281%2F25%2B1%2F15%29%29%28%285%2A5%2A3%29%2F%285%2A5%2A3%29%29

%282%2A5%2A5%2A3%29%2F%283%2B5%29

%282%2A5%2A5%2A3%29%2F%282%2A4%29

%285%2A5%2A3%29%2F4

75%2F4

highlight%2818%263%2F4%29, km%2Fhr

Answer by ikleyn(52787) About Me  (Show Source):
You can put this solution on YOUR website!
.
Her speed downstream is 20 + 5 = 25 km/h.

She spent  1%2F25 of an hour traveling 1 kilometer downstream.



Her speed upstream is 20 - 5 = 15 km/h.

She spent  1%2F15 of an hour traveling 1 kilometer for the returning trip upstream.



In all, she spent  1%2F25 + 1%2F15 hours = 3%2F75+%2B+5%2F75 = 8%2F75 hours.



The average rate is the total distance, which is 1 + 1 = 2 kilometers, divided by the total time, which is  8%2F25 hours:


    the average rate = 2%2F%28%288%2F75%29%29 = %282%2A75%29%2F8 = 75%2F4 = 18.75 kilometers per hour.    ANSWER

Solved.