[Javascript] Adding checkbox values

[alaneaston666 at hotmail.com (Alan Easton)]

Hello People,

I am trying something which I think is simple. I have a form with 9 
checkboxes and a text box. Each checkbox has a numeric value.

Now all I want to do is add them up when they get ticked, and the total 
value to appear in the text field, or subtract the value of the checkbox 
from the total if it is unchecked. I want this to happen automatically.

Here is what I am trying, but as yet it is not working.

**************CODE*************************
<script language="JavaScript"><!--
function calculate(what) {
    for (var i=1,answer=0;i<9;i++)
        answer += what.elements['Checkbox' + i].value - 0;
    what.answer.value = answer;
}
//--></script>

<form ID="Form2">
<INPUT type="checkbox" ID="Checkbox1" NAME="Checkbox1" VALUE="1" 
onchange="calculate(this.form)"><br>
<INPUT type="checkbox" ID="Checkbox2" NAME="Checkbox2" VALUE="1" 
onchange="calculate(this.form)"><br>
<INPUT type="checkbox" ID="Checkbox3" NAME="Checkbox3" VALUE="1" 
onchange="calculate(this.form)"><br>
<INPUT type="checkbox" ID="Checkbox4" NAME="Checkbox4" VALUE="1" 
onchange="calculate(this.form)"><br>
<INPUT type="checkbox" ID="Checkbox5" NAME="Checkbox5" VALUE="1" 
onchange="calculate(this.form)"><br>
<INPUT type="checkbox" ID="Checkbox6" NAME="Checkbox6" VALUE="1" 
onchange="calculate(this.form)"><br>
<INPUT type="checkbox" ID="Checkbox7" NAME="Checkbox7" VALUE="1" 
onchange="calculate(this.form)"><br>
<INPUT type="checkbox" ID="Checkbox8" NAME="Checkbox8" VALUE="1" 
onchange="calculate(this.form)"><br>
<INPUT type="checkbox" ID="Checkbox9" NAME="Checkbox9" VALUE="1" 
onchange="calculate(this.form)"><br>

<input type="text" name="answer" ID="Text13">
</form>
**************CODE*************************

Any help would be greatly appreciated.

Thanks,

Alan...

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