namely, what is 170% of 2.80? well, if we take 2.80 to be the 100%, what is the 170%?
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
2.80&100\\
x&170
\end{array}\implies \cfrac{2.80}{x}=\cfrac{100}{170}\implies \cfrac{2.80\cdot 170}{100}=x[/tex]