The density is given by:
[tex]\begin{gathered} \rho=\frac{m}{V} \\ where\colon \\ m=mass \\ V=Volume \end{gathered}[/tex]Therefore, she can calculate the density using the following formula:
[tex]Density=\frac{number_{\text{ }}of_{\text{ }}cookies}{volume_{\text{ }}of_{\text{ }}tin}[/tex]