Answer:
1. 
Date                   Account Title                                             Debit          Credit
Dec. 31             Bad debt expense                                    $9,000
                         Allowance for doubtful accounts                                 $9,000
Working 
= 3% * 300,000
= $9,000
2. 
Date                   Account Title                                             Debit          Credit
Dec. 31             Bad debt expense                                    $12,000
                         Allowance for doubtful accounts                              $12,000
Working
= 1% * total debt 
= 1% * (900,000 + 300,000)
= $12,000
3. 
Date                   Account Title                                             Debit          Credit
Dec. 31             Bad debt expense                                    $12,500
                         Allowance for doubtful accounts                              $12,500
Working
= 6% * Accounts receivable
= 6% * 125,000
= $7,500
As the Allowance account is in debit, it means that bad debt exceeded the allowance so this balance needs to be added to properly cater for bad debts. 
= 7,500 + 5,000
= $12,500