let's say the customer ordered "x" amount, which will be the 100% of muffins.
we know that "x" is 100%, and that 81 is 37.5%, what the dickens is "x"?
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
x&100\\
81&37.5
\end{array}\implies \cfrac{x}{81}=\cfrac{100}{37.5}\implies x=\cfrac{81\cdot 100}{37.5}[/tex]
