we simply divide the whole by the sum of the ratios, namely 28 by (3+4), and then distribute accordingly.
[tex] \bf \cfrac{GroupA}{GroupB}\qquad \cfrac{3\cdot \frac{28}{(3+4)}}{4\cdot \frac{28}{(3+4)}}\implies \cfrac{3\cdot 4}{4\cdot 4}\implies \cfrac{12}{16} [/tex]
