Vivid-View Theater sells books of theater tickets to its customers at $28 per book. Each book contains a certain number of tickets to future shows. During the current period, 1,000 books were sold for $28,000 cash and this amount was credited to Unearned Ticket Revenue. At the end of the period, the accountant for Vivid-View determined that $16,800 worth of tickets had been turned in at the box office. The appropriate adjusting entry at the end of the accounting period?