17.First calculate the cost of DVD,[tex]20-9.89=10.11[/tex]Then reduce it by 7.5%.[tex]10.11-10.11\cdot0.075\approx\boxed{9.35}[/tex]18.Average is,[tex]a=\displaystyle\Sigma_{i=1}^{n}\dfrac{a_i}{n}\Longrightarrow\dfrac{1}{n}\Sigma_{i=1}^{n}a_i[/tex]We know that n = 30 and,[tex]\Sigma_{i=1}^{30}a_i=3865.5[/tex]Hence,[tex]a=\dfrac{3865.5}{30}=\boxed{128.85}[/tex]19.So 2% of some number is 4586,[tex]x\cdot0.02=4586[/tex]Solve for x.[tex]x=\dfrac{4586}{0.02}=\boxed{229300}[/tex]Hope this helps.r3t40