The amount of the adjusting entry to record the estimated uncollectible receivables at the end of each year has been 1/4 of 1% of credit sales, which is the rate reported as the average for the industry. Credit sales and the year-end credit balances in Allowance for Doubtful Accounts for the past year.

Year Credit Sales Allowance for Doubtful Accounts
2005 $8,160,000 $8,520

Problem:
Determine the amount of (a) the addition to Allowance for Doubtful Accounts and (b) the accounts written off for year

For the adjusting entry itself, you want to do literally what they said: 1/4 of 1% of the credit sales. You have credit sales given, so you just need to do the math.

When doing the percent of sales method, the balance already in the allowance account is not taken into consideration. The number you get from the above calculation will be the number to put into your entry. (The balance in the allowance account after you get done with the entry will change, but that is not relevant to the entry itself.)

As for part (b), I don't see where you have the information to figure out what in total was actually written off.