AB co. began 2019 with a 500 balance in A/R and a 100 balance in the allowance for doubtful accounts. on 12/31/19, abc showed an ending balance in the allowance account if 110. bad debt expense for 2019 was 15 and 6000 was collected from customers. abc also showed a 3000 gross margin on 4000 of inventory sold (COGS) what amount of customer accounts was written off during 2019?