variables_5.scss

$width: 400px;
$padding: 30px;
$border-width: 1px;

.box {
    box-sizing: border-box;
    width: $width;
    padding: 30px;
    border: $border-width solid black;
}

.no-boxsizing .box {
    width: $width - ($padding * 2 + $border-width * 2);
}