The manager of Hal's Car Wash, Inc., was reviewing the utility bills of the business. His review showed that in May the bill was $3800 and in October the bill was $2000. If 600 cars were washed in May and 200 cars were washed in October, what is the mixed cost formula representing the company's utility bill? What is the approximate amount of the bill if 900 cars are washed? (Round any intermediate calculations to the nearest cent and your final answer to the nearest dollar.) Show all calculations.