first off, let's conver the mixed fraction to an improper one, and then check the proportions.
[tex] \bf \stackrel{mixed}{15\frac{3}{4}}\implies \cfrac{15\cdot 4+3}{4}\implies \stackrel{improper}{\cfrac{63}{4}}\\\\
------------------------------- [/tex]
[tex] \bf \cfrac{rise}{run}=\cfrac{1}{3}\qquad \qquad \cfrac{\quad rise\quad }{\stackrel{run}{\frac{63}{4}}}=\cfrac{1}{3}\implies \cfrac{\quad \frac{rise}{1}\quad }{\frac{63}{4}}=\cfrac{1}{3}
\\\\\\
\cfrac{rise}{1}\cdot \cfrac{4}{63}=\cfrac{1}{3}\implies \cfrac{4rise}{63}=\cfrac{1}{3}\implies rise=\cfrac{63}{4\cdot 3}
\\\\\\
rise=\cfrac{21}{4}\implies rise=5\frac{1}{4} [/tex]
