为什么这样程序不行？求修正
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>数组排序sort()</title>
</head>
<body>
<script type="text/javascript">
var scoreArr="小明:87;小花:81;小红:97;小天:76;小张:74;小小:94;小西:90;小伍:76;小迪:64;小曼:76";
var score=scoreArr.split(";");
var num=new Array();
for(var i=0;i<score.length;i++)
{
num=parseInt(score[i].slice(3,5));
}
for(var j=0;j<num.length;j++)
{
sum=sum+num[j];
}
var avg=sum/sum.length;
document.write("平均值:"+avg+"<br/>");
</script>
</body>
</html>
