A special donmno set contains all number pairs from double-0 fo double-8, with each number pair occurring only once.

Question

For example the following domino counts as 1-4 and 4-1. how many dominos are in the set?

Answer 1

If we start with 0, we have 9 possibilities for the other side, 0-8.

 

We then start with 1-1, because 1-0 is already accounted for, so there are 8 possibilities, etc.

 

9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 = 45 dominoes