Answer
$-3$
Work Step by Step
Note that:
$\dfrac{1}{125}=\dfrac{1}{5^3}=5^{-3}$
Thus, the given expression is equivalent to:
$$\log_5{\left(5^{-3}\right)}$$
Recall:
$$\log_a{\left(a^n\right)}=n$$
Thus,
$$\log_5{\left(5^{-3}\right)}=-3$$
