the ratio is simply one divided by the other, so let's firstly convert the mixed fractions to improper fractions.
[tex]\bf \stackrel{mixed}{6\frac{1}{4}}\implies \cfrac{6\cdot 4+1}{4}\stackrel{improper}{\cfrac{25}{4}}~\hfill \stackrel{mixed}{12\frac{1}{2}}\implies \cfrac{12\cdot 2+1}{2}\implies \stackrel{improper}{\cfrac{25}{2}} \\\\[-0.35em] \rule{34em}{0.25pt}\\\\ \cfrac{model}{actual}\qquad \cfrac{~~\frac{25}{4}~~}{\frac{25}{2}}\implies \cfrac{25}{4}\cdot \cfrac{2}{25}\implies \cfrac{25}{25}\cdot \cfrac{2}{4}\implies 1\cdot \cfrac{1}{2}\implies \cfrac{1}{2}\implies \stackrel{ratio}{1~:~2}[/tex]
