JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <div class="box">
  <div class="one">1/3</div>
    <div class="two">2/3</div>
  </div>
</div>

CSS

body
{border:1px dotted yellow;
margin:0;
padding:0;
width:100%;
height:100%;
background:brown;}

.container{
background:red;
width:250px;
height:100px;}

.box
{display:flex;
align-items:center;
justify-content:space-around;
}

.one
{background:green;
flex:1 1 auto;}

.two
{background:blue;
flex:2 1 auto;}