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4. The local garbage company charges $6 a month for garbage collection. It had been their practice to send out bills to their 100,000 customers at the end of each 2-month period. Thus, at the end of February it would send a bill to each customer for $12 for garbage collection during January and February Recently the firm changed its billing date: it now sends out the 2-month bills after one month's service has been performed. Bills for January and February, for example, are sent out at the end of January. The local newspaper points out that the firm is receiving half its money before garbage collection. This unearned money, the newspaper says, could be temporarily invested for one month at 1% per month interest by the garbage company to earn extra income. (a) [10 points] Draw cash flow diagrams in one year for previous billing model and current billing model, respectively (Hint: Interest is 1% per month, and compounding period is not specified, which implies monthly compounding.) (b) [20 points] Suppose the garbage company invests the money as described by the newspaper. In every billing cycle, after making a one-month temporary investment with unearned money, they withdraw principal to maintain regular operations in company, and leave the accrued interest in the same account. Compute how much extra income the garbage company could earn in one year. (Hint: See Example 4-17.) (Note: In previous billing model, there was no unearned money when company sent out bills; in new billing model, unearned money in one billing cycle is $6 for one customer.)