Answer: $18,000
Explanation:
Given that,
Began 2018 with a Normal balance = $5,000
Ended 2018 with a normal balance = $11,000
Unearned Revenue account was credited = $24,000
Revenue earned by professor in 2018 :
= Beginning unearned revenue + Advance payments - Ending unearned revenue
= $5,000 + $24,000 - $11,000
= $18,000
Therefore, $18,000 revenue earned by professor in 2018.