Debbie traveled by boat 5 miles upstream to fish in her favorite spot. Because of the 6-mph current, it took her 20 minutes longer to get there than to return. How fast will her boat go in still water? (Please round your answer to one decimal place.)
speed of boat --- x mph
speed upstream = x-6
speed downstream = x+6
5/(x-6) - 5/(x+6) = 20/60 = 1/3
times 3(x-6)(x+6)
15(x+6) - 15(x-6) = x^2 - 36
15x + 90 - 15x + 90 = x^2 - 36
x^2 = 216
x = √216 = appr 14.7 mph