an electrical company charges different rate to different classes of users. Domestic users pay a different rate to that paid by the commercial users. If the company Bolt-Elec charges 8.564 per kwh for the first 1000 kwh used, 6.475 per kwh for the next 9000kwh used and 4.250 per kwh for any additional kwh used. Dervive a function which can be used by Bolt-Elec to compute electricity bills for users.