Providing for doubtful accounts

At the end of the current year, the accounts receivable account has a debit balance of $$1,199,000$ and sales for the year total $$13,590,000.$

a$.$ The allowance account before adjustment has a debit balance of $$16,200.$ Bad debt expense is estimated at $\frac{1}{2}$ of $1\%$ of sales.

b$.$ The allowance account before adjustment has a debit balance of $$16,200.$ An aging of the accounts in the customer ledger indicates estimated doubtful accounts of $$51,800.$

c$.$ The allowance account before adjustment has a credit balance of $$4,700.$ Bad debt expense is estimated at $\frac{3}{4}$ of $1\%$ of sales.

d$.$ The allowance account before adjustment has a credit balance of $$4,700.$ An aging of the accounts in the customer ledger indicates estimated doubtful accounts of $$39,000.$

Determine the amount of the adjusting entry to provide for doubtful accounts under each of the assumptions $($a through d$)$ listed above.

a$.$ $

b$.$ $

C$.$ $

d$.$ $