[tex]\bf \cfrac{shirts}{price}\qquad \cfrac{3}{60},\cfrac{5}{100},\cfrac{8}{160},\cfrac{9}{180},\cfrac{12}{240}\implies \begin{array}{|c|ccccc|cc} \cline{1-6} x&3&5&8&9&12\\ \cline{1-6} y&60&100&160&180&240\\ \cline{1-6} \end{array}[/tex]
so let's pick two points off that shirts/price table, say 5,100 and 9,180
[tex]\bf (\stackrel{x_1}{5}~,~\stackrel{y_1}{100})\qquad (\stackrel{x_2}{9}~,~\stackrel{y_2}{180}) \\\\\\ slope = m\implies \cfrac{\stackrel{rise}{ y_2- y_1}}{\stackrel{run}{ x_2- x_1}}\implies \cfrac{180-100}{9-5}\implies \cfrac{80}{4}\implies \stackrel{\textit{average rate of change}}{\cfrac{20}{1}\implies \stackrel{\downarrow }{20}}[/tex]
