Justify elements with fix space (variable width)

Question

I have a container with a variable number of elements in it. The elements should be justified but with a fix space between (e.g. 20px). That means the width of

Answer 1

You can use display: table and display: table-cell for this:

.container {
  display: table;
  width: 100%;
  border-spacing: 10px 0;
  border-collapse: separate;
  background: palevioletred;
}
.container div {
  display: table-cell;
}
.container img {
  display: block;
  width: 100%;
  height: auto;
}

<div class="container">
  <div><img src="//dummyimage.com/200x100/000/CCC"></div>
  <div><img src="//dummyimage.com/300x100/000/CCC"></div>
  <div><img src="//dummyimage.com/400x100/000/CCC"></div>
</div>
<hr/>
<div class="container">
  <div><img src="//dummyimage.com/200x100/000/CCC"></div>
  <div><img src="//dummyimage.com/400x100/000/CCC"></div>
</div>
<hr/>
<div class="container">
  <div><img src="//dummyimage.com/600x100/000/CCC"></div>
</div>

Answer 2

If using Flexbox is an option, you could add flex: 1 to the flex items and also a margin property with a fixed value as follows:

EXAMPLE HERE

div.container { display: flex; }

div.container div {
  height: 50px; /* Just for demo */
  flex: 1;
  margin-left: 20px;
}

div.container :first-child { margin-left: 0; }

Actually, flex: 1 is a shorthand of flex-grow: 1; in this case.