first off, we'll convert the mixed fraction to "improper" and check about, keeping in mind that, there are 5280 feet in 1mile.
[tex]\bf \stackrel{mixed}{1\frac{1}{4}~mi}\implies \cfrac{1\cdot 4+1}{4}\implies \stackrel{improper}{\cfrac{5}{4}~mi}\\\\
-------------------------------[/tex]
[tex]\bf \begin{array}{ccll}
feet&miles\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
5280&1\\
f&\frac{5}{4}
\end{array}\implies \cfrac{5280}{f}=\cfrac{1}{\quad \frac{5}{4}\quad }\implies \cfrac{5280}{f}=\cfrac{\frac{1}{1}}{\quad \frac{5}{4}\quad }
\\\\\\
\cfrac{5280}{f}=\cfrac{1}{1}\cdot \cfrac{4}{5}\implies \cfrac{5280}{f}=\cfrac{4}{5}\implies \cfrac{5280\cdot 5}{4}=f\implies 6600=f[/tex]
