Question 26884
Since its total time of 9 hours you knwo that its first speed + the second.
{{{60/(x+3)-60/(x-3)=9}}}
60[(x+3)+(x-3)]=9[(x+3)(x-3)]
{{{60(2x)=9(x^2-9)}}}
{{{120x=9x^2-81}}}
{{{9x^2-120x-81=0,x}}}
Use the quadratic formula and simplfy you get
x=13.9777...
Round it out
Hence, the speed of the boat is 14mph in still water.
Paul.