Div expand to fit

by watson

HTML

<section>
    <div id="one"></div>
    <div id="two"></div>
</section>

CSS

section {
    width: 80%;
    height: 200px;
    background: aqua;
    margin: auto;
    padding: 10px;
}
div#one {
    width: 15%;
    height: 200px;
    background: red;
    float: left;
}
div#two {
    margin-left: 15%;
    height: 200px;
    background: black;
}