if we take 28 to be the 100%, what is 21 off of it?
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
28&100\\
21&p
\end{array}\implies \cfrac{28}{21}=\cfrac{100}{p}\implies p=\cfrac{21\cdot 100}{28}[/tex]
