let's say the bill without the tip is "x" amount, so that'd be the 100% of the bill, but we also know that 4.50 is the 18% of "x".
so if 4.50 is the 18% of "x", how much is "x"?
[tex]\bf \begin{array}{ccll} amount&\%\\ \cline{1-2} 4.50&18\\ x&100 \end{array}\implies \cfrac{4.5}{x}=\cfrac{18}{100}\implies 450=18x \\\\\\ \cfrac{450}{18}=x\implies 25=x[/tex]
