If we have two variables and can write a relationship between them as:
[tex]y=kx \\ \\ \\ Where: \\ \\ \text{k is the constant of proportionality}[/tex]
Then we say that [tex]y[/tex] varies directly as [tex]x[/tex] or [tex]y[/tex] is directly proportional to [tex]x[/tex]
In this case, let's say:
[tex]x:\text{The number of hours worked} \\ \\ y:\text{Total work fee}[/tex]
Then since we know that the fee for 5 hours of work is $255, then the constant of proportionality is:
[tex]k=\frac{255}{5} \\ \\ k=51[/tex]
Therefore, our model can be written as:
[tex]\boxed{y=51x}[/tex]
