SOLUTION: a mississippi riverboat can travel 75 kilometers downstream in three hours and can make the return trip in five hours. find the speed of the boat in still water.

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let speed of boat in still water = x
let speed of water = w
so we know:
with water, spped of boat = x+w
against water, speed of boat = x-w

now speed=distance/time

so, with water:
x+w = 75/3
x+w = 25
against water:
x-w = 75/5
x-w = 15

seeing as how we want x, lets get rid of w by adding the 2 equations together to give 2x = 40 --> x = 20km/hr

this is your answer.

jon.