codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>a</title>
<meta name=vs_defaultClientScript content="JavaScript">
<meta name=vs_targetSchema content="http://schemas.microsoft.com/intellisense/ie5">
<meta name="GENERATOR" content="Microsoft Visual Studio .NET 7.1">
<meta name=ProgId content=VisualStudio.HTML>
<meta name=Originator content="Microsoft Visual Studio .NET 7.1">
<script id="clientEventHandlersJS" language="javascript">
<!--

window.onload = function()
{
	document.getElementById("Checkbox1").onclick = Checkbox1_onclick;

}


function Checkbox1_onclick() 
{
	var t = document.getElementById("Text1");
	var tv = t.value;
	
	if(!isNum(tv)) return;
	
	var n = (this.checked)? +30: -30;
	
	t.value = parseFloat(tv) + n;
	
}


function isNum(arg) 
{
	return !isNaN(parseFloat(arg));
}


//-->
</script>
</head>
<body MS_POSITIONING="FlowLayout">
<INPUT type="checkbox" ID="Checkbox1" NAME="Checkbox1">
<INPUT type="text" ID="Text1" NAME="Text1" value="100">


</body>
</html>