JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <div class="left">fill free space (100% - right)</div>
    <div class="right">fixed width</div>
</div>

CSS

.container {
    width: 500px;
    background-color: bisque;
    height: 50px;
    display: table;
}
.container .left {
    background-color: burlywood;
    height: 50px;
    display: table-cell;
    width: 100%;
}
.container .right {
    background-color: chartreuse;
    width: 50px;
    height: 50px;
    display: inline-block;
    vertical-align: text-top;
}