What you are looking for is a function that results in the value of change
based on the value of a purchase with the amount tendered a constant $20.


Let c be the amount of change, and we want  where p is the
purchase amount.





I'm not sure what your book means by a function table, unless it is a list of
values for the independent variable (p in this case) against the value of the
function at that point.  Here's a fragment of such a table:




p
c


3.50
16.50


4.00
16.00


4.50
15.50


5.00
15.00