Providing for doubtful accounts

At the end of the current year, the accounts receivable account has a debit balance of $$910,000$ and sales for the year total $$10,320,000.$

a$.$ The allowance account before adjustment has a debit balance of $$12,300.$ Bad debt expense is estimated at $\frac{3}{4}$ of $1\%$ of sales.

b$.$ The allowance account before adjustment has a debit balance of $$12,300.$ An aging of the accounts in the customer ledger indicates estimated doubtful accounts of $$39,400.$

c$.$ The allowance account before adjustment has a credit balance of $$9,200.$ Bad debt expense is estimated at $\frac{1}{2}$ of $1\%$ of sales.

d$.$ The allowance account before adjustment has a credit balance of $$9,200.$ An aging of the accounts in the customer ledger indicates estimated doubtful accounts of $$76,400.$

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$.$ $