SOLUTION: A motorboat went 10 miles upstream in 1 hour. The return trip took only 30 minutes. Assume that the "water speed" and the current speed were constant during both parts of the trip.

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Let = the speed of the boat in still water
Let = the speed of the current
For the trip upstream:
(1)
For the trip downstream:
(2)
given:
mi
mi
hr
hr
upstream:


------------------------
(1)
(1)
(1)
(1)
and
(2)
(2)
(2)
(2)
-------------------------------
Subtract (2) from (1)
(1)
(2)
(3)


Now I can rewrite (1)
(1)
(1)
(1)
mi/hr
and, since
(1)
(1)
(1) mi/hr
The speed of ther boat in still water is 15 mi/hr
The speed of the current is 5 mi/hr
check:
(2)



OK