body {
  background-color: lightgray;
  font-family: Arial, Helvetica, sans-serif;
  }


#container {
  width: 1100px;
  margin: 0px auto;
  border: 4px solid gray;
}
  header {
    background-color: red;
    height: 100px;
    width: 100%;
    margin: 0px;
    text-align: center;
    line-height: 100px;
    color: antiquewhite;
    border-bottom: 2px solid black;
}
  nav ul li {
  float: left;
  list-style: none;
  margin: 20px;
  line-height: 10px;
}
a {text-decoration: none;}
ul {text-align: center; display: inline-block;}
#content{
    float: left;
    width: calc(80% - 20px);
    background-color:green;
    min-height: 500px;
    padding: 10px;
}
nav {
    background-color:pink;
    text-align:center;
}
.article {
color: white;
margin-top: 15px;
margin-bottom: 15px;
border-bottom: 1px solid white;
padding-bottom: 20px;
}
aside{
float: left;
/*Usando la función calc*/
width: calc(20% - 20px);
background-color: coral;
min-height: 500px;
padding: 10px;
}
/*Quinto Paso: Modicar el Footer */
footer {
background-color: black;
color: white;
text-align: center;
height: 30px;
line-height: 30px;
}
.clearfix{
clear:both;
}
* {
  margin: 0px;
  padding: 0px;
}