Question 131830
The current in a stream moves at a speed of 3mph. A boat travels 45 miles upstream and 45 miles downstream in a total time of 8 hours. What is the speed of the boat in still water? 
----------------------------
Let the speed of the boat in still water be "b"
----------------------------
Upstream DATA:
Distance = 45 miles ; Rate = b-3 mph ; Time = d/r = 45/(b-3) hrs.
-----------------------------
Downstream DATA:
Distance = 45 miles ; Rate = b+3 mph ; Time = d/r = 45/(b+3) hrs
--------------------------
EQUATION:
time + time = 8 hrs
45/(b-3) + 45/(b+3 = 8
45(b+3) + 45(b-3) = 8(b^2-9)
90b = 8b^2-72
8b^2-90b-72 = 0
(b-12)(8b+6)=0
Positive Solution:
b=12 mph (boat speed in still water)
============================
Cheers,
Stan H.