so he had a gross of $700, now 700 - 275 is 425, since 275 is what his weekly salary is, then the 425 is the 5.25% from the sales.
let's say "x" is the total amount for the sales, namely the 100%, but we also know that 5.25% of that is 425, the leftover from the 700 bucks minus his salary, what is "x" anyway?
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
x&100\\
425&5.25
\end{array}\implies \cfrac{x}{425}=\cfrac{100}{5.25}\implies x=\cfrac{425\cdot 100}{5.25}[/tex]
