Domino

By systematically checking for all tiles in a double-six set (0-0 through 6-6), we identify: 0-0, 0-1, 0-2, 0-3, 0-4, 0-5, 0-6; 1-1, 1-2, 1-3, 1-4, 1-6; 2-2, 2-3, 2-4, 2-5, 2-6; 3-3, 3-4, 3-5, 3-6; 4-4, 4-5, 4-6; 5-5, 5-6; 6-6. The only tile missing from the set is the [1, 5] domino.

The domino [0, 0] is missing from the given set of 27 dominoes.

By systematically going through all 28 possible domino combinations in a standard double-six set and comparing them to the 27 dominoes visible in the image, the domino [0,3] (blank-three) appears to be missing.

After systematically identifying all 27 dominoes visible in the image, I mapped out all combinations from 0-6. The domino [4, 4] (double-four) is not present in the set. All other 27 unique dominoes from the standard 28-tile double-six set are accounted for.

I inspected and tallied the 27 visible tiles and found every unordered pair from 0–6 except the tile with pips 3 and 5, so the missing domino is [3,5].

By systematically identifying each of the 27 dominoes in the image and comparing to the complete set of 28, the missing one is [5,6].

After cataloging all 27 dominoes visible in the image and comparing them against the complete double-six set of 28 tiles, the only missing combination is [2,5]. All tiles involving 2 are present except [2,5] (verified: [0,2], [1,2], [2,2], [2,3], [2,4], [2,6] are all visible). Similarly, for the number 5, all tiles [0,5], [1,5], [3,5], [4,5], and [5,6] are present, leaving [2,5] as the single absent tile.