How many dominos is in a set of dominos?

Use the formula (n+1)(n+2)/2, where n is the number on the highest double in the set. For a double 12 set: 13 x 14 / 2 = 91. For a double 9 set: 10 x 11 / 2 = 55 For a double 6 set: 7 x 8 / 2 = 28