TextBox enable Disable on checkChange

protected void rptrItem_ItemDataBound(object sender, RepeaterItemEventArgs e)
  {
  if (e.Item.ItemType == ListItemType.Item || e.Item.ItemType == ListItemType.AlternatingItem)
  {
  CheckBox chk = (CheckBox)e.Item.FindControl("chkbxItem");
  TextBox txt = (TextBox)e.Item.FindControl("txtQuantity");
  txt.Attributes.Add("onkeypress", "return isNumberKey(event)");
  txt.Attributes.Add("onpaste", "return false;"); 
  chk.Attributes.Add("onclick", "if(this.checked) {document.getElementById('" + txt.ClientID + "').disabled = !this.checked;document.getElementById('" + txt.ClientID + "').style.backgroundColor ='white';document.getElementById('" + txt.ClientID + "').focus();}else{document.getElementById('" + txt.ClientID + "').style.backgroundColor ='CCCCCC';document.getElementById('" + txt.ClientID + "').disabled = true;}");
   

  }
  }