How to make the right margin twice the size of the left margin

Question

I have a fixed-size inner div within a variable-width outer div.

#outer { width: 100%; }
#inner { width: 80px; }

<div id="outer">
  <div id="inner"></div>
</div>

I want the inner div to be placed inside outer div as follows:

[Npx of empty space][80px fixed-size inner div][2Npx of empty space]

where N is some number. As the window size changes, N will change, but the right will remain twice the size of the left. Is it possible to do with CSS?

Answer 1

As an alternative to Mr Lister's positioning option we can use calc on the margins.

#outer {
  width: 100%;
}
#inner {
  width: 80px;
  height: 35px;
  background: rebeccapurple;
  margin-top: 1em;
  margin-left: calc((100% - 80px) / 3);
  margin-right: calc((100% - 80px) / (3 * 2));
}

<div id="outer">
  <div id="inner">
  </div>
</div>