Die Anzahl der Jungen ist um$20\mathrm{\%}\backslash \; 20\backslash \%\backslash$kleiner als die Anzahl der Mädchen, also ist:

$j=(100\mathrm{\%}-20\mathrm{\%})\cdot m\Rightarrow \boxed{{\displaystyle j=\mathrm{0,8}\cdot \text{m}}}\backslash \; j=(100\backslash \%-20\backslash \%)\backslash cdot\; m\backslash Rightarrow\backslash \; \backslash boxed\{j=0\{,\}8\backslash cdot\; \backslash text\{m\}\}$ und:

$\boxed{{\displaystyle j=\text{m}-\frac{1}{5}\text{m}}}\backslash hspace\{39mm\}\backslash \; \backslash boxed\{j=\backslash text\{m\}-\backslash dfrac\{1\}\{5\}\backslash text\{m\}\}$