A firm offers terms of $1/10,$ net $30.$

a$.$ What effective annual interest rate does the firm earn when a customer does not take the discount? $($Do not round intermediate calculations and enter your answer as a percent rounded to $2$ decimal places, e$.$g$.,32\mathrm{.}16.)$

b$.$ What effective annual interest rate does the firm earn if the discount is changed to $2$ percent? $($Do not round intermediate calculations and enter your answer as a percent rounded to $2$ decimal places, e$.$g$.,32\mathrm{.}16.)$

c$.$ What effective annual interest rate does the firm earn if the credit period is increased to $50$ days? $($Do not round intermediate calculations and enter your answer as a percent rounded to $2$ decimal places, e$.$g$.,32\mathrm{.}16.)$

d$.$ What effective annual interest rate does the firm earn if the discount period is increased to $15$ days? $($Do not round intermediate calculations and enter your answer as a percent rounded to $2$ decimal places, e$.$g$.,32\mathrm{.}16.)$