in plain and short, we simply divide 27 by (5+4) and distribute accordingly.
[tex] \bf \cfrac{girls}{boys}\qquad \cfrac{5}{4}~\hspace{7em}\cfrac{5\cdot \frac{27}{5+4}}{4\cdot \frac{27}{5+4}}\implies \cfrac{5\cdot 3}{4\cdot 3}\implies \cfrac{\stackrel{\stackrel{girls}{\downarrow }}{15}}{12} [/tex]
