we know the bill is 35.40, so that's 100% of the bill, and Dimitri wants to pay 20% of that,
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
35.40&100\\
x&20
\end{array}\implies \cfrac{35.40}{x}=\cfrac{100}{20}\implies 35.40\cdot 20=100x
\\\\\\
\cfrac{35.40\cdot 20}{100}=x[/tex]
