Answer :

original price of shirts  = $ 20
new price of shirts =  $17
decrease in price =  $20 - $17  = $ 3
% decrease =        3/20 x 100 =  15%
[tex]\begin{array}{ccc}\$20&-&100\%\\\\\$17&-&p\%\end{array}\\\\\\cross\ multiply\\\\20p=17\cdot100\ \ \ /:20\\\\p=\frac{1700}{20}\\\\p=85\ (\%)\\\\100\%-85\%=15\%\leftarrow Answer[/tex]