algebra
posted by jjj .
A motorvoat goes 36 miles upstream on a river whose current is running at 3 miles per hour. The trip up and back takes 5 hours. What is the speed of the boat ( assuming that it maintains a constant speed relative to the water)?

t going upstream
(5  t) going downstream
speed upstream = (v3)
speed downstream = (v+3)
36 = t(v3)
36 = (5t)(v+3)
36 = t v  3 t
t = 36/(v3)
36 = [5  36/(v3)] (v+3)
36 = 5(v+3)  36(v+3)/(v3)
(21  5 v)(v3) = 36(v+3)
check, multiply out and solve quadratic for v
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