Answer
$800$ electric pencil sharpeners
Work Step by Step
Replacing $f(x)$ with $25$ results to
\begin{array}{l}\require{cancel}
f(x)=20+\dfrac{4000}{x}
\\\\
25=20+\dfrac{4000}{x}
.\end{array}
Using the properties of equality to isolate $x$ results to
\begin{array}{l}\require{cancel}
25-20=\dfrac{4000}{x}
\\\\
5=\dfrac{4000}{x}
\\\\
x(5)=\left( \dfrac{4000}{x} \right)x
\\\\
5x=4000
\\\\
x=\dfrac{4000}{5}
\\\\
x=800
.\end{array}
Hence, $800$ electric pencil sharpeners must be produced.