Edit in JSFiddle

<div class="BFC">
    <div class="box"></div>
    <div class="box"></div>
</div

.BFC {
        border: 5px solid #f00;
        width: 300px;
    }
 
    .box {
        border: 5px solid blue;
        width:100px;
        height: 100px;
        float: left;
    }