in plain and short, the whole class is 100%, so we first split it by the ratios, namely by 3+2, so 100/(3+2) and then give 3 pieces to the girls and 2 pieces to the boys, let's do so,
[tex]\bf \cfrac{girls}{boys}\qquad 3:2\qquad \cfrac{3}{2}\qquad \qquad \cfrac{3\cdot \frac{100}{3+2}}{2\cdot \frac{100}{3+2}}\implies \cfrac{3\cdot 20}{2\cdot 20}\implies \cfrac{60}{40}[/tex]
