simple math really. a simplified example:
<form action="" method="POST">
enter a number: <input type="text" name="number1" size=2><br>
chosse another number: <select name="number2">
<option>1
<option>2
<option>3
<option>4
<option>5
</select><br>
<input type="submit" name="submit" value="add these together">
</form>
<?
if (isset($_POST['submit']))
{
$total = $_POST['number1'] + $_POST['number2'];
echo $_POST['number1'] . ' + ' . $_POST['number2'] . ' = <b>' . $total . '</b>';
}
?>