Answer:
sales revenue for the period: $  143,900
operating expenses:               $   78,000
Explanation:
We solve for sales using the account recievable identity:
beginning account receivable + sales - collection = ending account receivable
12,330 + sales - 137,920 = 18,310
sales = 137,920 + 18,310 - 12,330 = 143,900
Then, for operating expenses, we have a prepaid expenses thus unexpired and therefore, not expenses under accrued accounting.
we solve like this:
beginning prepaid expenses    19,800
payment on expenses              84,990
total expenses payment          104,790
We now subtract the prepaid (unexpired) to get the amount accrued for the period:
104,790 - 26,790 = 78,000