Work Step by Step
Each domino has two parts and each part can have 0 to 6 pips. The total possible combinations excluding doubles is $7\times6$=42 However, due to the symmetry, 12 is the same as 21 etc. so we only need half of 42 which is 21. We also need to add the 7 doubles (00,11,22,33,44,55,66) to 21, so there are 28 dominoes in a complete set with no repetitions.