we know the diameter of the pizza is 20", that means the radius of it is half that or 10".
[tex]\bf \textit{area of a circle}\\\\ A=\pi r^2~~ \begin{cases} r=radius\\[-0.5em] \hrulefill\\ r=10 \end{cases}\implies A=\pi 10^2\implies A=100\pi \\\\\\ \stackrel{\textit{ divided by 8}}{\cfrac{100\pi }{8}\implies \cfrac{25\pi }{2}}\qquad \qquad \stackrel{\textit{how much for two slices of that?}}{\cfrac{25\pi }{2}+\cfrac{25\pi }{2}\implies \cfrac{50\pi }{2}}\qquad \approx \qquad 78.54[/tex]
