ABC Company agreed upon a ten-month, $18,000, 4% interest-bearing note receivable from XYZ Corporation on April 1, 2009 for a sale of a piece of equipment. In other words, ABC gave XYZ the equipment in exchange for a longer-term receivable. Assuming all necessary adjusting entries were made at year end December 31, 2009 and interest will be received upon maturity, the entry ABC makes on the notes maturity date would include a:

debit to interest revenue for $60

credit to interest receivable for $540

debit to interest expense for $600

credit to note payable for $18,000

The answer is "credit to interest receivable for $540" but I am not sure where they are getting the $540 from, thanks in advance!