Answer:
15 mph = rate of boat in still water
Step-by-step explanation:
D = RT
Let R = rate of boat in still water
Rate down stream = R + 5
Time down stream = 20/(R + 5)
Rate upstream = R - 5
Time upstream = 20/(R - 5)
Total time = 20/(R + 5) + 20/(R - 5) = 3
          20(R - 5) + 20(R + 5) = 3(R + 5)(R - 5)
           20R - 100 + 20R + 100 = 3R^2 - 75
                 3R^2 + 40R - 75 = 0
                 (3R + 5)(R - 15) = 0
                   R = -5/3  or  15
Rate cannot be negative, so R = 15 mph = rate of boat in still water Â
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