How to make a div fill a remaining horizontal space?

Question

I have 2 divs: one in the left side and one in the right side of my page. The one in the left side has fixed width and I want the one of the right side to fill the remaining space.

#search {
  width: 160px;
  height: 25px;
  float: left;
  background-color: #ffffff;
}

#navigation {
  width: 780px;
  float: left;
  background-color: #A53030;
}

<div id="search">Text</div>
<div id="navigation">Navigation</div>

Answer 1

This seems to accomplish what you're going for.

#left {
  float:left;
  width:180px;
  background-color:#ff0000;
}
#right {
  width: 100%;
  background-color:#00FF00;
}

<div>
  <div id="left">
    left
  </div>
  <div id="right">
    right
  </div>
</div>