say that "x" is the 100%, and we also know that 900 is the 4 and 1/2% of that, namely 4.5% then.
so if 900 is the 4.5% of "x", how much is "x"?
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
x&100\\
900&4.5
\end{array}\implies \cfrac{x}{900}=\cfrac{100}{4.5}\implies x=\cfrac{900\cdot 100}{4.5}[/tex]
