Answer
See below.
Work Step by Step
If $x$ is the speed of the boat and $y$ is the speed of the river, then our equations are: $(x+y)2.5=100$ and $(x-y)3=100$. So $x+y=40$ and $x-y=\frac{100}{3}$.
So $x=40-y$. then $x-y=(40-y)-y=\frac{100}{3}\\40-2y=\frac{100}{3}\\-2y=-\frac{20}{3}\\y=\frac{10}{3}$
Then $x=40-\frac{10}{3}=\frac{110}{3}$
