To calculate the density of a rock, you can use the formula:
\[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} \]
In this case, the mass of the rock is 13.5 grams, and the volume is 5 mL. Plugging in the values:
\[ \text{Density} = \frac{13.5 \text{ g}}{5 \text{ mL}} = 2.7 \text{ g/mL} \]
Therefore, the density of the rock is 2.7 g/mL.