Nine..................when multiplied with any number from 2 - 10....................the single digits and double digits of that product add up to nine.......

9 * 2 = 18                      1 + 8 = 9
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