"When I taught middle years Mathematics, I found myself intrigued by the different ways students solved puzzles. Classic puzzles, such as the following problem, use letters to represent numbers which can be displayed as simple calculation problems in disguise. One must replace each letter by a single 0 to 9 digit. The same digit must be used to represent the same letter. So, if you believe that 4 is being used to represent the letter A, then 4 must be used for all A’s in the same problem. Of course, one must use a different digit for different letters. After one substitutes all the letters for the numbers, a perfectly valid calculation problem will result."