Kim Davis has decided to buy a cellular phone, but she is unsure regarding which rate plan to select. The "regular" plan charges a fixed fee of $55 per month for 1,000 minutes of airtime plus $0.33 per minute for any time over 1,000 minutes. The "executive" plan charges the fixed fee of $100 per month for 1,200 minutes of airtime plus $0.25 per minute over 1,200 minutes.
a. If Kim expects to employ the phone for 21 hours per month, which plan should she choose?
b. At what level of employ would Kim be indifferent between two plans?