The examples in the text show a unique characteristic (think:

The examples in the text show a unique characteristic (think: positive or negative sign) associated with how the PRESENT VALUE (PV) of a monetary sum (when given) should be expressed when you are solving for future value (FV) of that sum.

What is that characteristic?  What questions would you ask a mortgage lender if you used the Time Value of Money calculations to copare loan terms with what you "expect" to pay (or owe) based on your calculations.  If you've never considered a home loan, substitute a car loan or (dare I say it ...) a student loan.