<div id="wrap">
  <div id="header"></div>
  <div id="nav"></div>
  <div id="aside"></div>
  <div id="container">
      <div id="section_a"></div>
      <div id="section_b"></div>
      <div id="section_c"></div>
  </div>
  <div id="footer"></div>
</div>

* {
    padding: 0;
    margin: 0;
}
body {
    background-color: #E1F5FE;
}
#wrap {
    display: flex;
    flex-wrap: wrap;
    width: 1200px;
    margin: 0 auto;
}
#header {
    width: 100%;
    height: 100px;
    background-color: #B3E5FC;
}
#nav {
    width: 100%;
    height: 100px;
    background-color: #81D4FA;
}
#aside {
    width: 30%;
    height: 780px;
    background-color: #4FC3F7;
}
#container {
    width: 70%;
}
#section_a {
    width: 100%;
    height: 260px;
    background-color: #29B6F6;
}
#section_b {
    width: 100%;
    height: 260px;
    background-color: #03A9F4;
}
#section_c {
    width: 100%;
    height: 260px;
    background-color: #039BE5;
}
#footer {
    width: 100%;
    height: 100px;
    background-color: #0288D1;
}
@media (max-width: 1220px){
    #wrap {
        width: 96%;
    }
    #container{
        display: flex;
        flex-wrap: wrap;
    }
    #section_b {
        width: 50%;
        height: 520PX;
    }
    #section_c {
        width: 50%;
        height: 520px;
    }
}   
@media (max-width: 768px){
    #wrap {
        width: 100%;
    }
    #aside {
        width: 30%;
    }
    #section_a {
        height: 390px;
    }
    #section_b {
        width: 100%;
        height: 390PX;
    }
    #section_c {
        display: none;
    } 
}
@media (max-width: 480px){
    #aside {
        width: 100%;
        height: 200px;
    }
    #container {
        width: 100%;
    }
    #section_a {
        width: 100%;
        height: 430px;
    }
    #section_b {
        width: 100%;
        height: 150px;
    } 
}