Question 8175
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.