* {
  box-sizing: border-box;
}

div.a {
  text-align: center;
}

.menu {
  float: left;
  width: 20%;
  }

.menuitem {
  padding: 8px;
  margin-top: 7px;
  border-bottom: 1px solid #f1f1f1;
}

.nav {
  float: left;
  width: 20%;
 }

.main {
  float: left;
  width: 60%;
  padding: 0 20px;
  overflow: hidden;
}

.article {
  float: left;
  width: 60%;
  padding: 0 20px;
  overflow: hidden;
}

.right {
  background-color: lightblue;
  float: left;
  width: 20%;
  padding: 10px 15px;
  margin-top: 7px;
}

@media only screen and (max-width:800px) {
  /* For tablets: */
  .main {
    width: 80%;
    padding: 0;
  }
  .article {
    width: 80%;
    padding: 0;
  }
  .right {
    width: 100%;
  }
                                         }

@media only screen and (max-width:500px) {
  /* For mobile phones: */
  .menu, .article, .main, .right {
    width: 100%;
  }
                                        }