On December 1st, 2005, $7500 was received for services to be performed from December 1, 2005 until May 31, 2006. Make the December 31, 2005 adjusting journal entry if the unearned fees at the end of December 2005 was $4200.

Help with this? I got the answer but am somewhat stumped. I got:

$3300 debit to Unearned Fees
$3300 credit to Fees Earned.

What I'm confused about is isn't this adjusted over a 6 month period? I don't see how unearned fees could be at $4200 at this period. $1250 was the entry I got for this. Help here?