in 2008 , subsequent to the allied acquisition , we recorded a provision for doubtful accounts of $ 14.2 million to adjust the allowance acquired from allied to conform to republic 2019s accounting policies . we also recorded $ 5.4 million to provide for specific bankruptcy exposures in 2008 . restricted cash and restricted marketable securities as of december 31 , 2010 , we had $ 172.8 million of restricted cash and restricted marketable securities . we obtain funds through the issuance of tax-exempt bonds for the purpose of financing qualifying expenditures at our landfills , transfer stations , and collection and recycling facilities . the funds are deposited directly into trust accounts by the bonding authorities at the time of issuance . as the use of these funds is contractually restricted , and we do not have the ability to use these funds for general operating purposes , they are classified as restricted cash in our consolidated balance sheets . in the normal course of business , we may be required to provide financial assurance to governmental agencies and a variety of other entities in connection with municipal residential collection contracts , closure or post- republic services , inc . notes to consolidated financial statements , continued .
concentration of credit risk financial instruments that potentially subject us to concentrations of credit risk consist of cash and cash equivalents , trade accounts receivable and derivative instruments . we place our cash and cash equivalents with high quality financial institutions . such balances may be in excess of fdic insured limits . to manage the related credit exposure , we continually monitor the credit worthiness of the financial institutions where we have deposits . concentrations of credit risk with respect to trade accounts receivable are limited due to the wide variety of customers and markets in which we provide services , as well as the dispersion of our operations across many geographic areas . we provide services to commercial , industrial , municipal and residential customers in the united states and puerto rico . we perform ongoing credit evaluations of our customers , but do not require collateral to support customer receivables . we establish an allowance for doubtful accounts based on various factors including the credit risk of specific customers , age of receivables outstanding , historical trends , economic conditions and other information . no customer exceeded 5% ( 5 % ) of our outstanding accounts receivable balance at december 31 , 2010 or 2009 . accounts receivable , net of allowance for doubtful accounts accounts receivable represent receivables from customers for collection , transfer , recycling , disposal and other services . our receivables are recorded when billed or when the related revenue is earned , if earlier , and represent claims against third parties that will be settled in cash . the carrying value of our receivables , net of the allowance for doubtful accounts , represents their estimated net realizable value . provisions for doubtful accounts are evaluated on a monthly basis and are recorded based on our historical collection experience , the age of the receivables , specific customer information and economic conditions . we also review outstanding balances on an account-specific basis . in general , reserves are provided for accounts receivable in excess of ninety days old . past due receivable balances are written-off when our collection efforts have been unsuccess- ful in collecting amounts due . the following table reflects the activity in our allowance for doubtful accounts for the years ended december 31 , 2010 , 2009 and 2008: .
as of december 31 , 2010 what was the ratio of the restricted cash and restricted marketable securities to the allowance for doubtful accounts
the balance at end of year of 2010 is $ 50.9 ; the balance at end of year of 2009 is $ 55.2 ; the balance at end of year of 2008 is $ 65.7 ; our receivables are recorded when billed or when the related revenue is earned , if earlier , and represent claims against third parties that will be settled in cash . restricted cash and restricted marketable securities as of december 31 , 2010 , we had $ 172.8 million of restricted cash and restricted marketable securities .
2010 2009 2008
balance at beginning of year $ 55.2 $ 65.7 $ 14.7
additions charged to expense 23.6 27.3 36.5
accounts written-off -27.9 ( 27.9 ) -37.8 ( 37.8 ) -12.7 ( 12.7 )
acquisitions - - 27.2
balance at end of year $ 50.9 $ 55.2 $ 65.7