Answer and Explanation:
In the first situation, the journal entry is
Cash Dr $1,600
To Unearned revenue $1,600
(Being the unearned revenue is recorded)
For this we debited the cash as it increased the assets and credited the unearned revenue as it also increased the liabilities
The adjusting entry is
Unearned Service Revenue XXXXX
To Service Revenue XXXXX
(Being the adjusting entry is recorded)
If this entry is not recorded than it would leads to understated of revenue and overstated of liabilities