In an equation made up of words you may use contractions in words that are not next to an operation sign or a comparison sign. In your example, all of the words would have to be uncontracted because "original" is next to the comparison sign and "discount" is next to the operation sign and the following comparison sign. You do not leave any spaces between the operation sign and the words. You would use (ed)4 for the boxes (if they are squares) and space them as if they are what they represent -- in this case a number. If the sentence expression must be divided at the end of the line, you must divide first of all before a comparison sign, just as you would with an equation composed of numbers.