Answer and Explanation:
According to the scenario,journal entry of the given data are as follow:- 
Journal Entry 
30 Sept Cash A/c Dr. $21,800 
 To Sales A/c $20,000 
 To Sales tax payable A/c $1,800  (20,000 × 9%) 
 ( Being cash sales is recorded)
30 Sept Cost of goods sold A/c Dr. $12,000 
 To Inventory A/c $12,000 
 ( Being September 30th sale cost is recorded)
15 Oct Sales tax payable A/c Dr. $1,800 
 To Cash A/c $1,800 
 ( Being payment of tax is recorded)