JSFiddle - React, Tailwind, and code Playground

HTML

<div class="block"><div class="left">1</div><div class="right">2</div></div>

<div class="block"><div class="newleft">3</div><div class="right">4</div></div>

CSS

div {
    display:inline-block;
    border: solid 1px #ccc;
}
.left {
    width:30%;
    padding:20px
}
.newleft {
    width:calc(30% - 44px); //40 отсутпы, 4 бордеры
    padding:20px;
}
.right {
    width:70%;
}
.block {
  display:block;
      float:none;
}