For a non-resident:
[tex]C_{nr}=8d+2.5d=10.5d[/tex]
For a resident:
[tex]C_{r}=114+1d[/tex]
We want to know the number of days [tex]d[/tex] such that the two costs are equal ie [tex]\{d|C_{nr}(d)=C_{r}(d)\}[/tex]
To do this set the two costs equal to each other:
[tex]C_{nr}=C_{r}[/tex]
[tex]10.5d=114+1d[/tex]
And solve for d:
[tex]10.5d-1d=114+1d-1d[/tex]
[tex]9.5d=114[/tex]
[tex]d=\frac{114}{9.5}[/tex]
[tex]d=12[/tex]
The costs will be equal when a resident and a non-resident each use the beach for 12 days.