Using Document.write To Write A Variable

I'm pretty new to JavaScript and am having a problem. I thought what I am doing should work but it isn't.

Basically I have a form that people can put a quantity into. e.g. A, B, C.

I have a JavaScript function called comput that assigns values e.g. A = 5, B = 7, C = 9. Here is the start of the script:

<script language="JavaScript"><!--
function compute(form){
var A = form.A.value * 5;
var B = form.B.value * 7;
var C = form.C.value * 9;

I then declare a variable to add them up:

var ans = A+B+C;

return;
}

</script>

This part of the script works fine. Now I want to write the ans variable. So I use:

<script>
<!--
document.write ("Your total is "+ans+"")
// --></script>

But it isn't working. Any ideas?