How do you calculate the rate of change of the volume of a cone at a given height and radius when the radius is decreasing and the height is increasing?

i have to calculate the partial derivatives, right? but then do i add the values?

like V = partial derivative height + partial derivative radius??
It would help if you gave example numbers of some sort, but here is the general solution.
The radius $r$ is changing at the rate of $r'$, and the height $h$ is changing at the rate of $h'$. The volume $V$ has a rate of change of $V'$. 

Now we know that $V = (\frac13\pi) r^2 h$. If you take the derivative of that, then you get (using product rule):

$V' = \frac13\pi\frac{d}{dt}(r^2h) = (\frac13\pi)(2rr'h + r^2h')$
All you have to do is plug in your current $r$ and $h$ values, and the rate of changes $r'$ and $h'$.

Just to make sure it is clear, $x'$ is the derivative of $x$.
The Radius of a cylinder is decreasing at a constant rate of 1 in per minute. The volume remains constant 415 cubic inches.  at the instant when the radius of the cylinder is 4 inches, what is the rate of change of the height?