Question 321379
Let r be the rate of the stream.

Then, upstream the boat travels at a rate of 5-r

Downstream, it travels at a rate of 5+r

Since d=rt, then t=d/r.

The trip takes 9 hours altogether, so we have:

{{{8/(5+r)+8/(5-r)=9}}}

Solving for r gives us {{{r=sqrt(145)/3=4.01}}} km/hr