in short, we simply divide the whole 300 by (3+7), to get in a 3:7 ratio, and then distribute accordingly.
[tex]\bf 300\qquad 3:7\qquad \cfrac{3}{7}~\hspace{5em}\cfrac{3\cdot \frac{300}{3+7}}{7\cdot \frac{300}{3+7}}\implies \cfrac{3\cdot 30}{7\cdot 30}\implies \cfrac{90}{210}\implies 90:210[/tex]
