To do this, we substitute the corresponding values of $x$ into the function $f(x)$.
For 1999, $x = 99$ because $x = 90$ corresponds to 1990. So, we substitute $x = 99$ into the function:
$$
f(99) = -154.41 + 39.38 \ln 99
$$
After calculating, we find that $f(99)
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