Domino

By systematically cataloging all visible dominoes in the image and comparing against the standard 28-tile double-six set (which contains all combinations of 0-6), the domino with sides 2 and 4 is not present among the 27 dominoes shown. All other 27 unique combinations are accounted for in the image.

By systematically checking the 28 unique tiles of a double-six domino set against the 27 tiles present in the image, all doubles except [5, 5] were identified: [0,0], [1,1], [2,2], [3,3], [4,4], and [6,6] are all visible. Since there are 27 tiles and all other combinations are accounted for, the double-five tile is the missing one.

I inspected the set of 27 tiles and the double-six (6-6) tile is not present among them; every other pair 0–0 through 5–5 and the mixed tiles are visible, so the missing domino is 6 and 6.

By systematically identifying all visible domino tiles in the image, I can account for 27 unique dominoes from the standard double-six set. The complete set should contain 28 tiles: [0,0], [0,1], [0,2], [0,3], [0,4], [0,5], [0,6], [1,1], [1,2], [1,3], [1,4], [1,5], [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], and [6,6]. After cataloging the visible tiles including [0,0] (vertical double-blank), [6,6] (horizontal double-six), [3,3], [4,4], [5,6], [4,6], [0,6], [2,5], [3,5], and others, the only tile not present is the [3,4] domino. This leaves 3 appearing on 7 tiles instead of 8, and 4 appearing on 7 tiles instead of 8, which confirms [3,4] is the missing domino.