If the density of a solid sphere decreases with its radius, at what radius is the density same as the average density?

Question

Suppose the density of a solid sphere is given by the following equation $$\rho=R-r$$

where $r \leq R$. This means the density of the sphere decreases as we go from its center to its surface

We know the total mass of the sphere is $M$, and total volume is $V=\frac{4}{3}\pi R^3$, the average density is $\rho_{avg}=\frac{M}{V}$

Then, at what $r$ value does $\rho =\rho_{avg}$?