well, if she bought 3 lamps at 40 bucks a pop, that's 120 bucks, but she paid 129.90, so the extra 9.90 is the tax.
now if we take the 120 as the 100%, what is 9.90 off of it in percentage?
[tex]\begin{array}{ccll} amount&\%\\ \cline{1-2} 120 & 100\\ 9.90& x \end{array} \implies \cfrac{120}{9.90}=\cfrac{100}{x}\implies \cfrac{120}{9.9}=\cfrac{100}{x} \\\\\\ 120x=990\implies x=\cfrac{990}{120}\implies x=8.25[/tex]
