Explanation:
The journal entries are shown below:
1	Bad debt expense	$18,000  
               To Allowance for doubtful account  $18,000
(Being the bad debt expense is recorded)
2	Allowance for doubtful account	$350  
                              To Accounts receivable  $350
(Being the written off amount is recorded)
3	Accounts receivable	$200  
                          To Allowance for doubtful account  $200
[Being account reinstated]  
Cash	$200  
                To Accounts receivable  $200
(Being the payment is received)
4	Cash	$400  
         Allowance for doubtful account	$200  
                          To Accounts receivable  $600
(Being the amount received is recorded)