Runge-Kutta-Fehlberg Method Problem Suppose all infected individuals remained in the population to spread the disease. A more realistic proposal is to introduce a third variable $z(t)$ to represent the number of individuals who are removed from the affected population at a given time $t$ by isolation, recovery and consequent immunity, or death. This quite naturally complicates the problem, but it can be shown that an approximate solution can be given in the form 
$x(t) = x(0)e^{\frac{-k1}{k2} z(t)}$    and $y(t)=m-x(t)-z(t)$,
where $k_1$ is the infective rate, $k_2$ is the removal rate, and $z(t)$ is determined from the differential equation
$z'(t)=k_2(m-z(t)-x(0)e^{\frac{-k1}{k2} z(t)})$.
Find an approximation to $z(30)$,$y(30)$, and $x(30)$, assuming that $m=100,000$,$x(0)=99,000$, $k_1$ = $2\times 10^{-6}$, and $k_2=10^{-4}$.
Now, the question would've been easier for me if there's only one variable, but I don't know how to make it work with three variables on Maple. 
So can anyone help me with this?

The differential equation you wrote for $z(t)$ only has one dependent variable $z$.  Everything else is a constant.  Once you have $z(30)$, you can plug that into the equations for $x(t)$ and $y(t)$.
