Answer:
the journal entry to record warranty expense is:
Dr Warranty expense 30,000
Cr Warranty liability 30,000
the journal entry to record actual expenses related to product warranties:
Dr Warranty liability 10,000
Cr Cash (or inventory, or wages payable) 10,000
Depending on what type of costs are incurred by the company, the account credited will vary, e.g. if units are replaced, then inventory must be credited, or if units are repaired and only labor is used, then wages payable or cash should be credited. Since the question doesn't give us a lot of details, I credited cash.