Answer: The adjusting entry is as follows:
Debit Deferred Revenue $1800
Credit Rent Revenue $1800
Explanation: In this scenario, it says that $3600 cash has been received for Dec and Jan. As of yet, the company has not yet earned the revenue.
On Dec 31, the company would have earned the revenue. Thus, we would debit deferred revenue and credit rent revenue of $1800, since 1/2 if that amount has been earned.