$size: xs sm md lg xlg auto;
$i: 0;

@each $col in 0.75rem, 0.85rem, 1rem, 2.2rem, 4.2rem, auto{
  $i: $i + 1;

  .mt-#{nth($size, $i)} {
    margin-top: $col;
  }
  .mb-#{nth($size, $i)} {
    margin-bottom: $col;
  }
  .ml-#{nth($size, $i)} {
    margin-left: $col;
  }
  .mr-#{nth($size, $i)} {
    margin-right: $col;
  }
}