JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent">
    <div class="left">
        L
    </div>
    <div class="right">
        R
    </div>
</div>

CSS

.parent {
    background: maroon;
    max-width: 500px;
}
.left {
    float: left;
    background: green;
    width: 40px;
    opacity: 0.7;
}
.right {
    width: calc(100% - 40px);
    background: blue;
    float: left;
}