Kim daivs has decided to purchase a cellular phone, but she is answer about which rate plan to select. The " regular " plan charges a fixed fee of 55% per month for 1000 minutes of airtime plus %0.33 per minute for any time over 1000 minutes. The executive plan charges a fixed fee of %100 per month 1200 minutes of airtime plus %25 per minute over 1200 minutes.
a. If kim excepts to use the phone for 21 hours per month, which plan should she select?
b. At what level of use would kim be indifferent between the two palns?