Let the number be $x$. Then, $x-16 \%$ of $x=42$ $\Leftrightarrow x-\frac{16}{100} x=42 \Leftrightarrow x-\frac{4}{25} x=42$ $\Leftrightarrow \frac{21}{25} x=42 \Leftrightarrow x=\left(\frac{42 \times 25}{21}\right)=50$.
Explanation as extracted from the printed page; notation may be imperfect.