SOURCE

<div class="container">
  <div class="box1">
         I dont have the width
    </div>
  <div>
</div>

.container{
        margin:0 auto;
        width: 500px;
        height:300px;
        background-color:green;
        border: 1px #ccc solid;
        margin-top: 60px;
        overflow:hidden;
}
.box1{
    background-color:red;
    width:auto;
    margin-left: -20px;
    
}