Answer:
The journal entries are as follows:
August 12,
Accounts receivable A/c Dr. $4,000
To service Revenue $4,000
(recording of services provided )
August 16,
Sales allowance discount A/c Dr. $490
To Accounts receivable $490
(recording of sales allowance)
August 20,
Cash [$4,000 - $490] A/c Dr. $3,510
To Accounts receivable $3,510
(recording of cash collection)