<div class="table">
  <div class="cell left">
    <div>Left as Tcell</div>
  </div>
  <div class="cell right">
    <div>Right as Tcell</div>
  </div>
</div>

<div class="flex">
  <div class="left">Left with Flex</div>
  <div class="right">Right with Flex</div>
</div>

.table {
  display: table;
  width: 100%;
  margin-bottom: 2em
}

.cell {
  display: table-cell;
}

.left {
  background: #d00;
}

.right {
  background: #0d0;
  text-align: right /* only needed for the table version */
}

.cell div {
  display: inline-block;
  background: #aaa;
  padding: 1em
}

.flex {
  display: flex;
  justify-content: space-between;
}

.flex div {
  padding: 1em
}