Prepaid insurance, beginning balance 700, payments for insurance during the period, 2100, prepaid insurance, ending, 800.
Would it be a debit 100 and a credit 100

No. First, for an entry you can never simply say a "debit of x and a credit of x." ALL entries have equal debits & credits so that doesn't mean a whole lot if you don't say what accounts you're using. So I don't know what account you want to debit and which to credit.

It looks like you are attempting to just add 100 to the original 700 to get 800. If you made payments for insurance during the period, what will that do to the balance of the account?

Then by saying "prepaid insurance, ending," they are meaning that's the balance it ended at and you have to get the account to that balance. The difference is what expired. But you have to get the correct balance prior to the entry by using that 2100 that was paid on insurance first.

For some "missing number" problems like this, you can use algebraic expression to solve it since you should know which numbers are added or deducted.
700+x-2100=800

Not only do you not need algebra to do this... and are probably better off not using it since people need to learn how to balance t accounts, but you've also got it wrong.

doesn't the 2100 represent the amortized portion? I read "paid" as "expensed", did you read it as addition to the prepaid expense account so 700+2100-x=800?

Amortized portion? You're off on a different road.

It gives the balance of prepaid insurance and the additional payments are added to that account. It's not expensed until it expires. Insurance is generally paid up front and expires as time goes by.

"Paying" for something has never implied an expense. That's not an accrued basis.

I see, thanks for correcting me
By the way, is it wrong if I use the term "amortize" to refer to the portion of the prepaid insurance that is expired?

The following is the unadjusted trial balance of P.D. trading on December 31, 2007, the end of the accounting period.

Cash P 10,000.00
Accounts Receivable 40,000.00
Allowance for Bad Debts P 400.00
Notes Receivable 12,000.00
Inventory, January 1 22,000.00
Furniture and Equipment 25,000.00
Accumulated Depreciation 10,000.00
Land 115,000.00
Accounts Payable 60,000.00
Notes Payable-bank debt in 2 years 20,000.00
Mortgage Payable-due in 5 years 50,000.00
P.D. Capital 99,000.00
Sales 360,000.00
Sales Returns and Allowance 11,000.00
Purchases 280,000.00
Freight In 6,200.00
Salaries 36,000.00
Rent Expense 24,000.00
Insurance 4,800.00
Other Operating Expenses 12,000.00
Interest Expense 1,800.00
Interest Income 600.00
Total P 600,000.00 P 600,000.00

Additional information for adjustments:

a. Estimated bad debts are 2 ½% of accounts receivable.
b. A note receivable P8,000.00, 60-day 6% is dated Nov. 16, 2007.
c. The furniture and equipment were acquired on Jan. 1, 2005 and are estimated to last two years from December 31, 2007.
d. The note payable to bank is 15% note dated Sept. 1, 2007.
e. Accrued interest on mortgage is P1,500.00.
f. Accrued salaries 1,200.00.
g. Prepaid rent P8,000.00
h. Unexpired insurance P600.00.
I. Prepaid interest expense P300.00.
j. Prepaid interest income P100.00.
k. Inventory, December 31, 2007, P62,440.00.

Delfin, please start your own thread instead of tagging onto someone else's. That just confuses people. Second, please go to the top of this forum and read the red homework guidelines. We do not just do your homework for you, and you need to show us your attempts at doing it first.

i see, thanks for correcting me
btw, is it wrong if i use the term "amortize" to refer to the portion of the prepaid insurance that is expired?

Theoretically that term is not incorrect since it can mean to write off something, but at least in the U.S. we don't generally use that term as applied to insurance. In Canada I know they use that term for plant assets (only cause I had that discussion with someone once). I use amortization as applied to intangible assets, and also for things like loans.