JSFiddle - React, Tailwind, and code Playground

HTML

<div id="a">70%</div>
<div id="b">30%</div>

CSS

div {
    background-color: #666666;
    color: white;
    height: 100px;
    float: left;
}

#a {
    margin-left: 30px;
    width: -webkit-calc(70% - 30px - 15px);
}

#b {
    margin-left: 30px;
    margin-right: 30px;
    width: -webkit-calc(30% - 30px - 15px);
}