Solution;
[tex]\begin{gathered} \text{Avery already has the following;} \\ 6+4+8=18 \\ \text{If she hopes to bike a total of 25, then she would have} \\ \text{Sunday}=\text{Total}-18 \\ \text{Sunday}=25-18 \\ \text{Sunday}=7 \end{gathered}[/tex]Avery should bike 7 miles on Sunday