My favorite feature of the HP calculator program is the solver function. With this function, I write equations and let the computer solve for any value in the equation. (See Through the Looking Glass, page 34, for an example of using Solve for Break-Even Analysis.)
One of Solver's useful features is sharing variables among equations. For example, an equation calculating fxdcost could be written. Computing a value for this variable will make it immediately available to the average cost equation or any other equation using fxdcost. Variable sharing allows breaking difficult equations into smaller more understandable equations.