[Tex/LaTex] fp calculation variable

Question

I want to do some calculations in a LaTeX document; the result of the first is used in the next calculation. I figured out that the fp package should do the job. Here is an example:

\FPeval{\result}{round(400*1.06,0)}%
$400+6\%=\result$\\
\FPeval{\result}{round(424*1.06,0)}%
$424+6\%=\result$

The first \result should be stored in a varable of some sort an fed into the next equation.

Answer 1

The fp package README mentions two examples and then also some warning/usage of macro names inside calculations:

 Example 1:
   The macro call
         \FPupn\result{17 2.5 + 17.5 - 2 1 + * 2 swap /} 
   is equivalent to
     \result := ((17.5 - (17 + 2.5)) * (2 + 1)) / 2
   and evaluates to
     \def\result{-3.000000000000000000}
   Afterwards the macro call
         \FPupn\result{\result{} -1 * 0.2 + sin 2 round}
                              ^^ the "{}" is necessary!
   is equivalent to
     \result := round_2(sin((\result * -1) + 0.2))
   and evaluates to
         \def\result{-0.06}
 Example 2:
   As "result" is an abbreviation of "\result{}" you may
   write
     \FPupn{result}{17 2.5 + 17.5 - 2 1 + * 2 swap /}
   and
     \FPupn{result}{result -1 * 0.2 + sin 2 round}
   instead leading to the same results.
   This is even true for other macro names using e.g. "x" for "\x{}"
   and so on. But be careful with it. We may introduce new constants
   in further versions overwriting these abbreviations.
 - fp-eval.sty:
     The following macros are public ones to be used in the document:
       \FPeval#1#2      % #1 := eval(#2) where eval evaluates the
                     expression #2
 ATTENTION: Do not use macro names with \. for its own
 Use only the name or the macro surrounded by (, and ) instead,
 i.e. do not write "\value{}" but "value" or "(\value)".
 This is needed to avoid problems with a prefix "-" of numbers.
 (I do not intend to write a more complex parsing routine in future.
  But if you do so, just send it to me.
 )

Here's following the package guidelines:

\documentclass{article}
\usepackage{fp}% http://ctan.org/pkg/fp
\begin{document}

\FPeval{\result}{round(400*1.06,0)}%
$400+6\%=\result$

\FPeval{\result}{round((result)*1.06,0)}%
$424+6\%=\result$

\end{document}