A TI-89 graphing calculator displays the behavior described by the Original Poster. If you enter "$7.99 + 8.25%" and evaluate it, you get 8.0725. On a TI-89, the % function is defined as taking one argument, which it divides by 100.

There is no "proper" definition of the % function; it is implemented differently on different calculators. In fact, the % function is commonly defined in two different ways on the *same* calculator. On a conventional non-programmable algebraic, the "%" key will perform either of two different functions, depending on the preceding operation.

For example, my cheap TI-30X Solar generates the following results:

For 7.99 + 8.25%, you get 0.6592 when you hit the % key

For 7.99 * 8.25%, you get 0.0825 when you hit the % key

For addition (and subtraction), the TI-30X assumes that X + Y% should be XY/100. This is the way that the Original Poster apparently wants the % key to work.

For multiplication (and division), the TI-30X assumes that X + Y% should be Y/100, which is a different function. The TI-89 defines the % function in this way only. On a programmable calculator, like the TI-89, you have to pick one or the other definition for %. You can't have it both ways, because it could lead to unpredictable results in programs.

On an HP-50G, the % key is defined totally differently from the TI-89. A 50G takes two arguments (not one), and calculates XY/100 (not Y/100). So %(7.99,8.25) is 0.66, which is the result that the Original Poster apparently expects. But in this case, the % function "eats" the 7.99 value, so you would have to use an extra ENTER to store it first.

On any graphing calculator, if you don't like the way that % is implemented, then it is trivial to write a simple program to replace it.

*Edited: 30 June 2008, 6:48 p.m. *