The first thing we have to know is that logistical growth is the per capita growth rate of a population that gets smaller and smaller as the size of the population approaches a maximum number of people. In the case of the zombie apocalypse, the dead would not count and the maximum population number would be the population of the U.S is 325,000,000.
[tex]\begin{gathered} L(x)=\frac{a}{1+be^{-rx}} \\ a=325,000,000\to\text{USA population} \\ r=0.2\to\text{growth rate} \\ x\to\text{time in days } \\ b\to\text{constant} \\ \approx \end{gathered}[/tex]For x = 0, they found 100 open graves so with this information we can find b
[tex]\begin{gathered} 100=\frac{325,000,000}{1+be^{-0.2(0)}} \\ 1+b=\frac{325,000,000}{100} \\ b=3,250,000-1 \\ b=3,249,999 \end{gathered}[/tex]Then the growth equation will remain
[tex]L(x)=\frac{325,000,000}{1+3,249,999e^{-0.2x}}[/tex]