a130/_sass/bourbon/functions/_grid-width.scss

@function grid-width($n) {
  @return $n * $gw-column + ($n - 1) * $gw-gutter;
}

// The $gw-column and $gw-gutter variables must be defined in your base stylesheet to properly use the grid-width function.
//
//  $gw-column: 100px;         // Column Width
//  $gw-gutter: 40px;          // Gutter Width
//
//  div {
//    width: grid-width(4);    // returns 520px;
//    margin-left: $gw-gutter; // returns 40px;
//  }