Use IF To Avoid Getting Anything Written After

I'm havin' a chart for interest, instalments and so on;

Interest [X%] Loan [Z crowns/dollars]
Loantime [Y years] Instalment [formula, which is "=Z/Y/12"]for each month.

Year, Month, Loan(left), Instalment, Total payment
1, Januari, =Z, =Instalment (constant), =Interest+Instalment

Month, Loan(left), Interest
Februari, =above-Instalment, =X%*Loan(left)/12 (rent is divided per year),

Instalment, Total payment
Constant, =rent+Instalment

and so on... as long as you want ^^ (it's quite messy, but try :D)

The question is as follows "What would a loan of 900'000kr with 9% interest rate totally cost over 30 years?"

[use "IF" to avoid getting anything written after the loan expires]

The question itself can be easily solved by just dragging the formulas down to 30 years and fill in the % and loan etc. But what I do not understand is the "IF" question. I'm supposed to use the IF function to instantly show the cost of the loan (total rent payment) after the 30 years? That's how I understood it. In that case... how do I do that? ^^