How would the accounting journal entry for this appear?

At the end of the month, the restaurant sold one kitchen oven purchased at the beginning of the month and reported a loss of $360

[Earlier in the problem it states that The restaurant acquired 4 identical kitchen ovens for $10,000. Each oven has a 5-year useful life with a total salvage (residual) value of 400.]

Earlier I got that the monthly depreciation was $160 so I'm also not sure if I should divide that to get the depreciation of one oven or just use that whole number.

First you need to calculate depreciation for one month for one oven to get the depreciation up to date.
So the cost of one oven is $2,500 ($10,000 / 4) minus $400 salvage value equals $2,100 depreciable base.

Now take $2,100 and divide it by 5 to get your depreciation for one year, which is $420 per year. To get your depreciation for one month take $420 depreciation per year times 1 divided by 12 months to get one month of depreciation, which is $35

So your adjusting entry will be:
Debit Depreciation Expense for 35
Credit Accumulated Depreciation for 35

This gets your depreciation up to date prior to the sale of one oven.