Example 4: In May 2006, Commonwealth Edison Company supplied electricity to residences for a monthly customer charge of $7.58 plus $0.08275 per kilowatt-hour (kWhr) for the first 400 kWhr
supplied in the month, and $0.06208 per kWhr for all usage over 400 kWhr in the month. (a) What is the
charge for using 300 kWhr in a month? (b) What is the charge for using 700 kWhr in a month? (c) If Cis
the monthly charge for x kWhr, develop a model relating the monthly charge and kilowatt-hours used.
That is, express C as a function of x.