Answer:
1 ) Debit Allowance for uncollectible accounts Adjustment $8,400 Credit Allowance for uncollectible accounts $8,400
2 a) Debit Doubtful Debt $7,560 Credit Accounts Receivable $7,560
b) Allowance for uncollectible accounts balance = $6,888
3 a) Debit Doubtful Debt $11,340 Credit Accounts Receivable $11,340
 b) Allowance for uncollectible accounts balance = $6,132
Explanation:
2021
Uncollected balance = $144,000- $102,000 = $42,000
Allowance for uncollectible Accounts = $42,000*20% = 8,400
2 b) Uncollectible accounts balance = $42,000- $7,560 =$34,440 *20%
                                           = $6,888
3 b) Uncollectible accounts balance = $42,000 - $11,340
                           =30,660*20%
                           =$6,132