Question

In a set of dominoes, each piece is marked with two numbers, one on each end. the pieces are symmetrical so that the two numbers are unordered. that is, the combination (2, 6) and (6, 2) are not distinguishable. how many pieces can be formed using the numbers 1, 2, ... , 12?

Answer 1

That would be the number of combinations of of 2 from 12 + the number of doubles ( 1 and 1 , 2 and 2 and so on).

= 12C2 + 12

= (12*11)/ 2 + 12

= 66 + 12 = 78 Answer