[tex]\bf \textit{volume of a cylinder}\\\\
V=\pi r^2 h\qquad
\begin{cases}
r=radius=\frac{diameter}{2}\\
h=height\\
----------\\
diameter=1\frac{1}{2}\\\\
r=\cfrac{1\frac{1}{2}}{2}\\\\
h=2\frac{1}{2}
\end{cases}[/tex]
so.. get the full volume of the cooler, by plugging in the given values
now, the cooler is only filled 3/4, how much is 3/4 of that? well, let's say you got a value V, 3/4 of that is , well [tex]\bf \cfrac{3}{4}\cdot V[/tex]
