GCD and Reduce to chain extension of column width classes

SassMeister-input.scss

// Sass v3.2.5

@function gcd($n,$d) {
    $numerator: if($n < $d,$d,$n);
    $denominator: if($n < $d,$n,$d);
    $remainder: $numerator;
	$last_remainder: $numerator;
	@while $remainder != 0 {
		$last_remainder: $remainder;
		$remainder: $denominator % $numerator;
		$denominator: $numerator;
		$numerator: $remainder;
	}
	@if $last_remainder != 0 {
		@return $last_remainder;
	}
}

@function reduce($n,$d) {
	$gcd: gcd($n,$d);
	@return ($n/$gcd,$d/$gcd)
}

$columns: 24;

@for $d from 1 through $columns {
    @for $n from 1 through $d {
		$ratio: reduce($n,$d);
		$x: nth($ratio,1); $y: nth($ratio,2);
		%span-#{$n}-#{$d} {
			@if $x < $n {
				@extend %span-#{$x}-#{$y};
			} @else {
				width: $n/$d*100%;
			}
		}
	}
}


.test {
    @extend %span-3-4;
}

.test2 {
	@extend %span-9-12;
}

SassMeister-output.css

.test2, .test {
  width: 75%; }