if the day care center started with 0 children ,we have that
[tex]\begin{gathered} \frac{3}{5}x+x=320\rightarrow \\ \frac{8}{5}x=320 \\ x=320\cdot\frac{5}{8} \\ x=200 \end{gathered}[/tex]It will reach it's limit when they get 200 children, so
[tex]t=\frac{200}{20}=10[/tex]so after 10 years it will reach full capacity