You need JavaScript for this (or atleast that's the most sensible solution).
set up your form like this:
Code:
<form>
Value 1: <input onchange="calc();" type="text" name="value1" id="value1" />
Value 2: <input onchange="calc();" type="text" name="value2" id="value2" />
etc.
</form>
<div id="equation"></div><input type="button" value="Calculate" onclick="calc();" />
<div id="total"></div>
and the JavaScript would be:
Code:
<head>
<script type="text/javascript">
calc() {
document.getElementById('equation').innerHTML =
document.getElementById('value1').text + ' x ' + document.getElementById('value2').text + ' - ' +
document.getElementById('value3').text + ' = ' + (document.getElementById('value1').text * document.getElementById('value2').text - document.getElementById('value3').text);
}
</script>
</head>
I hope that's enough for you to create your own version 'cause I don't feel like typing anymore lol