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 /* 5 is the number of list items and the number within $list */
$list: "0 0", "50px 0", "100px 0", "150px 0", "200px 0";

@for $i from 1 through 5 {
  li:nth-child(#{$i}) {
    $imgKey: random(5);
    $nth: nth($list, $imgKey);
    background-position: #{$nth};
  }
}