first off, let's convert the mixed fractions to "improper", keeping in mind that the water is increasing every passing hour by 1 and 1/4.
[tex]\bf \stackrel{mixed}{1\frac{1}{4}}\implies \cfrac{1\cdot 4+1}{4}\implies \stackrel{improper}{\cfrac{5}{4}}
\\\\\\
\stackrel{mixed}{3\frac{1}{2}}\implies \cfrac{3\cdot 2+1}{2}\implies \stackrel{improper}{\cfrac{7}{2}}\\\\
-------------------------------[/tex]
[tex]\bf \begin{array}{ccll}
feet&hours\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
\frac{5}{4}&1\\\\
\frac{7}{2}&h
\end{array}\implies \cfrac{\quad \frac{5}{4}\quad }{\frac{7}{2}}=\cfrac{1}{h}\implies \cfrac{5}{4}\cdot \cfrac{2}{7}=\cfrac{1}{h}\implies \cfrac{5}{2}\cdot \cfrac{1}{7}=\cfrac{1}{h}
\\\\\\
\cfrac{5}{14}=\cfrac{1}{h}\implies h=\cfrac{14\cdot 1}{5}\implies h=\cfrac{14}{5}\implies h=2\frac{4}{5}[/tex]
