Given: f(x)=log 5 x what is the value of f(125) and f(1/25)? Totally lost!

Question

I assume you mean f(x) = log (5x), not log 5 * x. If this is the case, then
f(125) = log 600
and
f(1/25) = log (1/5)
If the base of the logs is 10, then
log 600 = 2.778..