We will have to solve as follows:
*Since we know that the downstrean speed is 6mph, then we will want to know how much time will take to go 9 miles downstream, that is:
[tex]t=\frac{9\cdot1}{6}\Rightarrow t=1.5[/tex]Now, we know he takes 1.5 hours to move 9 miles downstream. With this information, we will solve for the speed he needs to move at in order to translate 4 miles uptream, that is:
[tex]v=\frac{4}{1.5}\Rightarrow v\approx2.67[/tex]He will then, move at aproximately 2.67 miles per hour while going upstream.
That is because he will cover the same ammount of time, but will cover a shorter distance.
