let's firstly convert the mixed fractions to improper fractions. and then let's make them with the same denominator by simply multiplying each other by the other's denominator.
[tex]\bf \stackrel{mixed}{3\frac{1}{4}}\implies \cfrac{3\cdot 4+1}{4}\implies \stackrel{improper}{\cfrac{13}{4}}~\hfill \stackrel{mixed}{3\frac{4}{5}}\implies \cfrac{3\cdot 5+4}{5}\implies \stackrel{improper}{\cfrac{19}{5}} \\\\[-0.35em] \rule{34em}{0.25pt}[/tex]
[tex]\bf \textit{now let's \underline{multiply each other by the other's denominator}} \\\\\\ \cfrac{13}{4}\cdot \cfrac{5}{5}\implies \cfrac{65}{20}\qquad \qquad \qquad \cfrac{19}{5}\cdot \cfrac{4}{4}\implies \cfrac{76}{20}~\hfill \cfrac{65}{20}<\stackrel{more}{\cfrac{76}{20}} \\\\\\ \stackrel{\textit{by how much?}}{\cfrac{76}{20}-\cfrac{65}{20}}\implies \cfrac{76-65}{20}\implies \cfrac{11}{20}\impliedby \textit{by that much}[/tex]
