calculate

If you have an HTML editor such as 1st Page 2000 (free athttp://www.evrsoft.com/download)you can paste this code into a new blank document, replace all occurances of &lt; with < and all occurances of &gt; with > and save it as test.htm you'll be able to see how it is working and modify it to suit your needs.

&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"&gt;

&lt;html&gt;
&lt;head&gt;
&lt;title&gt;Add Two Numbers&lt;/title&gt;

&lt;script language="JavaScript"&gt;

function NumOnly(event) {
&lt;!-- Make sure only numbers or a decimal point are entered //--&gt;
var charCode = (event.keyCode)
if (charCode &gt; 31 && (charCode &lt; 48 || charCode &gt; 57)) {
if (charCode !=46) {
return false;
}
}
}

function calculate() {
&lt;!-- Check if first box was left empty //--&gt;
if (document.myform.firstnumber.value == "" ) {
  &lt;!-- If so, focus on first box //--&gt;
  myform.firstnumber.focus();
  &lt;!-- and exit this function //--&gt;
  return false;
}

&lt;!-- Check if second box was left empty //--&gt;
if (document.myform.secondnumber.value == "" ) {
  &lt;!-- If so, focus on second box //--&gt;
  myform.secondnumber.focus();
  &lt;!-- and exit this function //--&gt;
  return false;
}

&lt;!-- Get first number from the form  (named 'myform') //--&gt;
var a=parseFloat(document.myform.firstnumber.value);

&lt;!-- Get second number from the form (named 'myform') //--&gt;
var b=parseFloat(document.myform.secondnumber.value);

&lt;!-- DO CALCULATION HERE - Here, a is added to b to give c //--&gt;
var c=a+b;

&lt;!-- Display some text, then the answer to the sum, then a reset button in the 'answer' span //--&gt;
document.getElementById("answer" ).innerHTML = '&lt;p&gt;'+a+' + '+b+' = '+c+'&lt;/p&gt;&lt;input type="button" value="Reset" onClick="resetform();"&gt;';
}

function resetform() {
   &lt;!-- Clear form //--&gt;
document.myform.reset();

&lt;!-- Change text in the 'answer' span //--&gt;
document.getElementById("answer" ).innerHTML = '&lt;p&gt;Enter two numbers & click &lt;b&gt;Answer&lt;/b&gt;.&lt;/p&gt;&lt;INPUT type="button" value="Answer" onClick="calculate();"&gt;';

&lt;!-- Focus on first box //--&gt;
myform.firstnumber.focus();
}

&lt;/script&gt;

&lt;/head&gt;
&lt;body&gt;

&lt;table width="300" cellpadding="0" cellspacing="0" border="0"&gt;
 &lt;tr&gt;
   &lt;td&gt;
     &lt;FORM name="myform"&gt;
       &lt;INPUT name="firstnumber" size="20" onkeypress="return NumOnly(event);"&gt;&lt;br&gt;&lt;br&gt;
       &lt;INPUT name="secondnumber" size="20" onkeypress="return NumOnly(event);"&gt;&lt;br&gt;&lt;br&gt;
       &lt;span id="answer"&gt;
     &lt;p&gt;Enter two numbers & click &lt;b&gt;Answer&lt;/b&gt;.&lt;/p&gt;
 &lt;INPUT type="button" value="Answer" onClick="calculate();"&gt;
   &lt;/span&gt;
 &lt;/FORM&gt;
   &lt;/td&gt;
 &lt;/tr&gt;
&lt;/table&gt;

&lt;/body&gt;
&lt;/html&gt;

Edit:

Just incase that's hard to follow, here's a simplified version of the same thing. It won't filter unwanted characters or swap buttons and clear the fields, etc., but with all that taken out it might be clearer:

&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"&gt;

&lt;html&gt;
&lt;head&gt;
&lt;title&gt;Add Two Numbers&lt;/title&gt;

&lt;script language="JavaScript"&gt;
function calculate() {
var a=parseFloat(document.myform.firstnumber.value);
var b=parseFloat(document.myform.secondnumber.value);
var c=a+b;
document.getElementById("answer" ).innerHTML = c;
}
&lt;/script&gt;

&lt;/head&gt;
&lt;body&gt;

&lt;table width="300" cellpadding="0" cellspacing="0" border="0"&gt;
 &lt;tr&gt;
   &lt;td&gt;
     &lt;FORM name="myform"&gt;
       &lt;INPUT name="firstnumber" size="20"&gt;&lt;br&gt;&lt;br&gt;
       &lt;INPUT name="secondnumber" size="20"&gt;&lt;br&gt;&lt;br&gt;
       &lt;INPUT type="button" value="Answer" onClick="calculate();"&gt;
 &lt;/FORM&gt;
 &lt;span id="answer"&gt;&lt;/span&gt;
   &lt;/td&gt;
 &lt;/tr&gt;
&lt;/table&gt;

&lt;/body&gt;
&lt;/html&gt;

Does that cover what you need to do?