Once upon a time, a teacher wrote a sequence of multiplication equations on the board: 9 × 1 = 7 9 × 2 = 18 9 × 3 = 27 9 × 4 = 36 9 × 5 = 45 9 × 6 = 54 9 × 7 = 63 9 × 8 = 72 9 × 9 = 81 9 × 10 = 90 9 × 11 = 99 9 × 12 = 108 The students, however, burst into laughter as they spotted a delibe…