var num=Math.floor(Math.random()*200)+250;
function dothis(){
    if(num>0){
        num=num-Math.floor(Math.random()*6);
        document.getElementById('cd').innerHTML=num;
        setTimeout(" dothis()",1500);
    }else{
        num=0;
        document.getElementById('cd').innerHTML=num;
    }
}
</script>