let's say the breakfast price was "x", so that's the 100%, and we know she gave 20% of that and that is 1.80.
so, 1.80 is 20%, and "x" is the 100%, what is "x"?
[tex]\bf \begin{array}{ccll} amount&\%\\ \cline{1-2} 1.80&20\\ x&100 \end{array}\implies \cfrac{1.80}{x}=\cfrac{20}{100}\implies \cfrac{1.8}{x}=\cfrac{1}{5} \\\\\\ 9.0=x\implies 9=x[/tex]
