If we subtract $\sqrt{5}$ from $\sqrt{125}$, the value is

Simplify $\sqrt{125}$ into its radical components:
$$\sqrt{125}=\sqrt{25\times 5}=5\sqrt{5}$$

Now subtract $\sqrt{5}$:

$$5\sqrt{5}-\sqrt{5}=4\sqrt{5}$$

Express $4\sqrt{5}$ under a single square root sign:

$$4\sqrt{5}=\sqrt{4^2\times 5}=\sqrt{16\times 5}=\sqrt{80}$$