Expand a div to fill the remaining width

Question

I want a two-column div layout, where each one can have variable width e.g.

Answer 1

Use calc:

.leftSide {
  float: left;
  width: 50px;
  background-color: green;
}
.rightSide {
  float: left;
  width: calc(100% - 50px);
  background-color: red;
}

  
a
  
b

The problem with this is that all widths must be explicitly defined, either as a value(px and em work fine), or as a percent of something explicitly defined itself.