so, we know both the rectangular prism and the cylinder got filled up to a certain height each, the same height say "h" cm.
we know the combined volume of both is 80 cm³, so let's get the volume of each, sum them up to get 80 then.
[tex]\bf \stackrel{\stackrel{\textit{volume of a}}{\textit{rectangular prism}}}{V=Lwh}~~ \begin{cases} L=length\\ w=width\\ h=height\\[-0.5em] \hrulefill\\ L=4\\ w=2\\ \end{cases}~\hspace{2em}\stackrel{\textit{volume of a cylinder}}{V=\pi r^2 h}~~ \begin{cases} r=radius\\ h=height\\[-0.5em] \hrulefill\\ r=1 \end{cases} \\\\[-0.35em] \rule{34em}{0.25pt}[/tex]
[tex]\bf \stackrel{\textit{prism's volume}}{(4)(2)(h)}~~+~~\stackrel{\textit{cylinder's volume}}{(\pi )(1)^2(h)}~~=~~80 \\\\\\ 8h+\pi h=80\implies h(8+\pi )=80\implies h=\cfrac{80}{8+\pi }\implies h\approx 7.180301999[/tex]
