SCSS operators

SCSS has a handful of standard math operators like +, -, *, /, and %.

Use simple math to calculate widths for an aside & article. For example:
.container { width: 100%; }

article[role="main"] {
  float: left;
  width: 600px / 960px * 100%;
}

aside[role="complementary"] {
  float: right;
  width: 300px / 960px * 100%;
}
The generated CSS looks like:
.container {
  width: 100%;
}

article[role="main"] {
  float: left;
  width: 62.5%;
}

aside[role="complementary"] {
  float: right;
  width: 31.25%;
}

For more information on Sass, see the Sass/SCSS reference.