JSFiddle - React, Tailwind, and code Playground

HTML

<div id="parent_div">
	<div id="left_div">A</div>
	<div id="right_div">
		<input type="button" value="ABC">
	</div>
</div>

CSS

#parent_div {
    display: -webkit-flex;
    display: flex;
}

#parent_div > div {
    -webkit-flex: 1 1 auto;
    flex: 1 1 auto
}

#left_div {
	width: 80%;
	min-width: 100px;
	height: 80%;
	background-color: #000;
    color: #FFF;
}

#right_div {
	width: 15%;
	min-width: 100px;
	font-size: 2rem;
	background-color: blue;
}