For the problem below, why can't you calculate the answer as 4% times the credit sales during 2017? (4% of gross AR)?

Thanks

3. Shore Co. provides for doubtful accounts based on 4% ofgross accounts receivable, The following data are available for 2017. Credit sales during 2017 Bad debt expense Allowance for doubtful accounts 1/1/17 Collection of accounts written off in prior years (customer credit was reestablished) Customer accounts written off as uncollectible during 2017 $4,400,000 57,000 17,000 8,000 30,000 What is the balance in Allowance for Doubtful Accounts at December 31, 2017