sass loops

@for $i from 1 through 6 {
    .grid-#{$i} {
        width: 100px*$i;
    }
}

$base-color: #036;

@for $i from 1 through 3 {
  ul:nth-child(3n + #{$i}) {
    background-color: lighten($base-color, $i * 5%);
  }
}