Concatenation of sass value+units. See also: http://stackoverflow.com/a/9862328

sass-var-concat.scss

// Set sizes without units
$basefont: 20;
$total-width-px: 1169;
$column-width-px: 72;

// Concatenation of units by addition results in conversion to string. This is bad
$basefont-px: $basefont+'px';  // = "20px"

// Conversion to pixels using multiplication
$basefont-px: $basefont*1px;  // = 20px;

// Conversion to percentages using multiplication
$column-width: ($column-width-px/($total-width-px/100))*1%;  // = 6.159110351%;