Yorkis Perez gave the bank a 90-day, 10% note for $7,200 on December 1, 2012.

What is the adjusting entry for this if the books are being closed on December 31, 2012?

It's a note payable by the way

You have to calculate one month of interest. The formula for interest is: Interest = Principal X Rate X Time

So Take your Principal times your rate times 1/12 to get one month worth of interest.

Your accounts to debit and credit are Interest Expense and Interest Payable.

Thank you!

Careful - your loan note is in days, so your interest calculation should be in days as well.