以下是随机显示广告的代码,想加入显示频率控制,例如把第一个广告列为70%的随机显示频率,请教该如何加?谢谢了

<script language="javascript">
a = 4
var slump = Math.random();
var talet = Math.round(slump * (a-1))+1;
function create() {
this.src = ''
this.border = ''
     this.alt = ''
     this.url = ''
}

b = new Array()
for(var i=1; i<=a; i++) { b[i] = new create() }

function flash(url_f,width_f,height_f){ //显示FLASH的HTML!
theOK='<table width='+width_f+' height='+height_f+' border="0" cellspacing="1" cellpadding="0" bgcolor="#000000" > <tr><td bgcolor="#FFFFFF"><object classid="clsid27CDB6E-AE6D-11cf-96B8-444553540000" codebase="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=5,0,0,0" width='+width_f+' height='+height_f+'><param name=movie value='+url_f+'><param name=quality value=high><param name="wmode" value="transparent"><embed src='+url_f+' quality=high pluginspage="http://www.macromedia.com/shockwave/download/index.cgi?P1_Prod_Version=ShockwaveFlash" type="application/x-shockwave-flash" width='+width_f+' height='+height_f+' wmode="transparent"></embed></object></td></tr></table>';
return theOK;
}

b[1].src = "http://www.bn163.net/468-60/468-050525.gif"
b[1].border = "0"
b[1].alt = "11"
b[1].url = "http://www.sohu.com/"


b[2].src = "http://www.bn163.net/468-60/20050305.gif"
b[2].border = "0"
b[2].alt = "22"
b[2].url = "http://www.sina.com"


b[3].src = flash("http://www.bn163.net/468-60/468-041201.swf","468","60"
b[3].border = ""
b[3].alt = ""
b[3].url = ""

b[4].src = flash("http://www.bntv.cn/swf/468.swf","468","60"
b[4].border = ""
b[4].alt = ""
b[4].url = ""

if (b[talet].url==""{
posb="<div align='center' valign='middle'>"
pose="</div>"
document.write(posb);
document.write(b[talet].src);
document.write(pose);
}
else{
var visa = "";
visa += '<a href='+b[talet].url+' target="_blank"><img src='+b[talet].src+' width="468" height="60" border="0"'+' alt='+b[talet].alt+'></a>';
document.write(visa);
}
</script>

每条广告加个权重值，设广告权重之和为X，根据X得到0~X间的随机数后，使每个广告的权重循环相加，直至和大于或等于X，这时最后一个相加的就是要显示的广告