# missing symbols in mathmode

I'm trying to insert a couple of numbered equations into a latex document, the equation is very basic yet all the normal operators =, + and - don't appear when I compile the document. I commented everything out except for the equation and still get the same output so I don't think it is as a result of anything before it in the document. This is the first document I'm writing using latex so chances are its something really basic!! the two equations are

$$Z = \frac{c_o - 4c_p}{c_{Si} + Rc_M } - 2$$


and

$$NC = 4 - 2Z$$


This is the header to the document

\usepackage{baththesis}
\usepackage[small,it]{caption}
\usepackage[usenames, dvipsnames]{xcolor}
\usepackage[sort&compress]{natbib}
\usepackage{multirow, array, textcomp, placeins, rotating, amsmath, amsthm, amssymb, acronym, graphicx, subfig, setspace}
\usepackage{isotope}% package for isotopes [#]{element}
\usepackage{fouriernc}
\usepackage{txfonts}
\usepackage[version=3]{mhchem} %package for checmical compositions ce{composition}

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Welcome to TeX.sx! A tip: If you indent lines by 4 spaces or enclose words in backticks , they'll be marked as code, as can be seen in my edit. You can also highlight the code and click the "code" button (with "{}" on it). – Guido Nov 13 '12 at 11:50
The package baththesis doesn't seem to be included in texlive full, and commenting it out doesn't solve the problem, so I suggest it can be removed from the MWE. – T. Verron Nov 13 '12 at 11:54
Commenting out \usepackage{fouriernc} does solve the problem (but changes the aspect of the equation as well). – T. Verron Nov 13 '12 at 11:57
It seems it depends by some interferences by fouriernc and txfonts. It is not present when one uses newtxtext but it is still present with newtxmath. Also inverting the order of loading the fouriernc and txtext solves it. – Guido Nov 13 '12 at 12:01
Loading both fouriernc and txfonts doesn't make sense. Use only either one of them. If you prefer Times to New Century Schoolbook, then try \usepackage{newtxtext,newtxmath} instead of \usepackage{txfonts} – egreg Nov 13 '12 at 12:29
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Loading both fouriernc and txfonts doesn't make sense. Use only either one of them.

If you prefer Times to New Century Schoolbook, then try

\usepackage{newtxtext,newtxmath}


\usepackage{txfonts}

The newtx` packages solve the spacing problems of the original TX fonts.