JSFiddle - React, Tailwind, and code Playground

HTML

<div style="position:relative">
    <div class='wrapper'>
      <div style="width: 70%">a</div>
      <div style="width: 20%">b</div>
      <div style="width: 10%">c</div>
      <div style="clear:both"></div>
   </div>
   <div class="constant-width">d<div>
</div>

CSS

.wrapper{
     margin-right: 100px;
 }
 .wrapper div{
     float:left;
    background: red
 }
 .constant-width{
     top: 0px; right:0px; position:absolute;
     width: 100px;
    background: blue
 }