title: Sass error when calculating with percent and pixels
body: <p>I want to be able  a responsive grid, that I can input my column and my padding value.</p>
  
  <pre><code>@mixin setColumnWidth($columns, $margin) {
  $totalColumn: 16;
  $totalMargin: ($margin*2)* $columns*1px;
  $totalWidth: calc(100% - #{$totalMargin})/$totalColumn;
  $columnWidth: $totalWidth*$columns;
  width: $columnWidth;
  margin-left: $margin;
  margin-right: $margin;
  float: left;}
  
  .articleCard {
  @include setColumnWidth(4, 16);
  }
  </code></pre>
  
  <p>But output is here</p>
  
  <blockquote>
    <p>Error: Undefined operation: "calc(100% - 0px)/16 times 3".<br>
    on line 37 of Users/mac/Documents/Projects/LeoX/New Version/HTML/scss/_myvalue.scss, in mixin <code>setColumnWidth</code></p>
  </blockquote>