# Surface Density of a Galaxy

The surface density of luminous matter in a galaxy has units of mass per area and obeys the equation

$D(r) = D_0 e^{-r/r_0},$

where $D_0$ is some constant with units of mass per area. Suppose we have some galaxy where $D_0 = 0.25 \text{ kg}/\text{m}$. What is $D(2r_0)$ in units of $\text{ kg}/\text{m}$, to the nearest thousandth?

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