sass mixins for image grid

imagegrid_mixin.scss

$roundness: 20px 0 20px 0;

@mixin clearfix {
  &:before,
  &:after {
    content: '';
    display: table;
  }
  &:after {
    clear: both;
  }
}

@mixin imagegrid($qty, $margin) {
  width: ((100% - (($qty - 1) * $margin))/$qty);
  &:nth-child(n) {
    margin-right: $margin;
    margin-bottom: $margin;
  }
  &:nth-child(#{$qty}n) {
    margin-right: 0;
    margin-bottom: 0;
  }
}

.grid {
	@include clearfix;
	.item {
		float: left;
		@include imagegrid(14, 1%);
	}
	img {
		display: block;
		border-radius: nth($roundness, 1);
		max-width: 100%;
	}
}