At year-end, December 31, 2017, Corolla Sales showed unadjusted balances of: $394,000 in Accounts Receivable; $13,800 debit in Allowance for Doubtful Accounts; and $1,940,000 in Sales. Uncollectible ac- counts are estimated to be 2.5% of sales.
Unadjusted balances at December 31, 2018, were Accounts Receivable, $514,000; Allowance for Doubtful Accounts, $1,000 credit; and Sales, $3,280,000. Corolla Sales changed the method of estimating uncollectible accounts to 4% of outstanding accounts receivable.
At December 31, 2019, the general ledger showed unadjusted balances of: $460,000 in Accounts Receivable; $700 debit in Allowance for Doubtful Accounts; and $3,400,000 in Sales. Corolla prepared an aging analysis on December 31, 2019, that estimated total uncollectible accounts to be $26,500.

Required 

Prepare the 2017, 2018, and 2019 year-end adjusting entries to estimate uncollectible accounts.
Analysis Component: On December 31, 2017, the unadjusted balance in Allowance for Doubtful Accounts was a debit of $13,800. What is the normal balance for the Allowance for Doubtful Accounts account? What would cause Allowance for Doubtful Accounts to have an unadjusted debit balance?