Answer
8 pounds for $\$$2.16 is a better buy
Work Step by Step
8 pounds for $\$$2.16 --> $\frac{\$2.16}{8\text{ pounds}} = 0.2700 \frac{\text{ dollars}}{\text{ pounds}}$
18 pounds for $\$$4.99 --> $\frac{\$4.99}{18\text{ pounds}} = 0.2772 \frac{\text{ dollars}}{\text{ pounds}}$
0.2700 < 0.2772