Answer :
well, she missed 3 throws only from all 20's, hmmm if we take 20(origin amount) to be the 100%, what's 3 off of it in percentage?
[tex]\begin{array}{ccll} amount&\%\\ \cline{1-2} 20 & 100\\ 3& x \end{array} \implies \cfrac{20}{3}~~=~~\cfrac{100}{x} \\\\\\ 20x=300\implies x=\cfrac{300}{20}\implies x=15[/tex]
