Generated by SassMeister.com.

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

@for $i from 1 through 6 {
  &.front:nth-child(#{$i}), &.back:nth-child(#{$i}) {
    $brick-bg: '../image/brick' + random(6) + '.jpg';
    background-image: url($brick-bg);
    background-size: 100% 100%;
  }
}

SassMeister-output.css

.front:nth-child(1), .back:nth-child(1) {
  background-image: url("../image/brick3.jpg");
  background-size: 100% 100%;
}

.front:nth-child(2), .back:nth-child(2) {
  background-image: url("../image/brick2.jpg");
  background-size: 100% 100%;
}

.front:nth-child(3), .back:nth-child(3) {
  background-image: url("../image/brick1.jpg");
  background-size: 100% 100%;
}

.front:nth-child(4), .back:nth-child(4) {
  background-image: url("../image/brick4.jpg");
  background-size: 100% 100%;
}

.front:nth-child(5), .back:nth-child(5) {
  background-image: url("../image/brick2.jpg");
  background-size: 100% 100%;
}

.front:nth-child(6), .back:nth-child(6) {
  background-image: url("../image/brick3.jpg");
  background-size: 100% 100%;
}