Two cells are shaped like cubes. Cell X has a side of 3 µm and cell Y a side of 9 µm…

Step 1. Cell X. Area $= 6 \times 3^2 = 54\ \mu\text{m}^2$, volume $= 27\ \mu\text{m}^3$, ratio $= \dfrac{54}{27} = 2$ per µm.

Step 2. Cell Y. Area $= 6 \times 9^2 = 486\ \mu\text{m}^2$, volume $= 729\ \mu\text{m}^3$, ratio $= \dfrac{486}{729} = 0.667$ per µm.

Step 3. Compare. X has 3 times as much surface for each µm³, because its side is 3 times smaller.