Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.1)
// Compass (v0.13.alpha.10)
// ----

%t1 { background-image: url('top.png'); }
%t15 { background-image: url('bottom.png'); }

@for $i from 1 through 50 {
  .thing-#{$i} {
    @if $i < 15 { @extend %t1; }
    @else { @extend %t15; }
  }
}

SassMeister-output.css

.thing-1, .thing-2, .thing-3, .thing-4, .thing-5, .thing-6, .thing-7, .thing-8, .thing-9, .thing-10, .thing-11, .thing-12, .thing-13, .thing-14 {
  background-image: url("top.png");
}

.thing-15, .thing-16, .thing-17, .thing-18, .thing-19, .thing-20, .thing-21, .thing-22, .thing-23, .thing-24, .thing-25, .thing-26, .thing-27, .thing-28, .thing-29, .thing-30, .thing-31, .thing-32, .thing-33, .thing-34, .thing-35, .thing-36, .thing-37, .thing-38, .thing-39, .thing-40, .thing-41, .thing-42, .thing-43, .thing-44, .thing-45, .thing-46, .thing-47, .thing-48, .thing-49, .thing-50 {
  background-image: url("bottom.png");
}