SOLUTION: in a river that flows 3 mph., a boat takes 1 hour longer to sail 36 miles upstream than return. find the speed of the boat in still water?

Question 310729: in a river that flows 3 mph., a boat takes 1 hour longer to sail 36 miles upstream than return. find the speed of the boat in still water?
Answer by ankor@dixie-net.com(22740) (Show Source): You can put this solution on YOUR website!
in a river that flows 3 mph., a boat takes 1 hour longer to sail 36 miles upstream than return. find the speed of the boat in still water?
:
Let s = boat speed in still water
then
(s+3) = speed downstream
and
(s-3) = speed upstream
:
Write a time equation; Time = dist/speed
:
Time downstream + 1 hr = time upstream
+ 1 =
Multiply equation by (s+1)(s-3), results:
36(s-3) + 1(s+3)(s-3) = 36(s+3)
:
36s - 108 + s^2 - 9 = 36s + 108
Combine the variables on the left and numbers on right
36s - 36s + s^2 = 108 + 108 + 9
:
s^2 = 225
s =
s = 15 km/hr is the boat speed in still water
:
:
Check solution by finding the times
Upstream 12 km/hr, downstream 18 km/hr
36/12 = 3 hrs
18/18 = 2 hrs
-------------
differ: 1 hr
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