One too many "F"s in that If, but nice logic...
'Course, I'd do it a little different, which might not make your professor happy. I'd use three cells for the whole thing: subtotal, discount, and total.
Subtotal is your current total.
Discount is Night Stalker's formula, except put the .9 back to .1.
Total is =if(discount="N/A",subtotal,(subtotal-discount))
Why? Because in the real world, all three numbers have importance to me. In a test, however, the instructor may want only what the instructor wants. I've been bitten by that before... sometimes, it pays more to know less.
Oh... I always use names in my spreadsheets as well, just because they're easier to track down when something goes rwogn...
Good luck!
*B*
Minister, etc. (ret.)