function  modem(){
 if(document.myfrom.mybox5.checked==true)
     {
       window.document.getElementById("modem_num").disabled=false;
       var a12 = window.document.getElementById("modem_a12").value;
       var total = 12 * a12;
       window.document.getElementById("modem_total").value = total;
    }
  else
      {
       window.document.getElementById("modem_num").disabled=true;
       window.document.getElementById("modem_total").value = 0;
 }
}