what we can do is divide the whole by the sum of the ratios, namely divide 35 by (4+3) and then distribute accordingly.
[tex]\bf \cfrac{boys}{girls}\qquad 4:3\qquad \cfrac{4}{3}\qquad \qquad \cfrac{4\cdot \frac{35}{4+3}}{3\cdot \frac{35}{4+3}}\implies \cfrac{4\cdot 5}{3\cdot 5}\implies \cfrac{20}{\underset{girls}{15}}[/tex]
