SOURCE

<!-- float-->
<div class="box1">
    <div class="left1">
左边部分
    </div>
    <div class="right1">
右边部分
    </div>
</div>
<!--flex-->
<div class="box2">
    <div class="left2">
左边部分
    </div>
    <div class="right2">
右边部分
    </div>
</div>

.box1{
    overflow: hidden;
}
.left1{
    float: left;
    background: red;
    width: 200px;
    height: 600px;
}
.right1{
    background: blue;
    margin-left: 210px;
    height: 200px;

}
.box2{
    display: flex;
}
.left2{
    width: 200px;
    height: 600px;
    background: yellow;
}
.right2{
    flex: 1;
    height: 200px;
    margin-left: 10px;
    background: green;
}