Explanation:
The adjusting journal entries are shown below:
On January 15
1. Cash A/c Dr $600 ($60 × 10 voice lessons)
To Unearned revenue A/c $600
(Being the cash is received)
On January 31
2. Unearned revenue A/c Dr $480 ($60 × 8 voice lessons)
To Service revenue A/c $480
(Being the unearned revenue is recorded)