How many dots should replace the question mark?

Concept: When a series of dominoes is grouped into consecutive pairs, add the total number of dots on both halves of every domino in a pair. These pair-totals themselves increase by a fixed step, and that step tells you the total required for the final pair.

Applying this to the given series:

1. First pair: the 1st domino shows 1 dot in total and the 2nd domino shows 2 dots in total, giving a pair-total of 1 + 2 = 3.

2. Second pair: the 3rd domino shows 2 dots in total and the 4th domino shows 4 dots in total, giving a pair-total of 2 + 4 = 6.

3. Third pair: the 5th domino shows 5 dots in total and the 6th domino shows 4 dots in total, giving a pair-total of 5 + 4 = 9.

4. The pair-totals 3, 6, 9 increase by 3 each time, so the fourth pair-total must be 9 + 3 = 12.

5. The 7th domino shows 7 dots in total (6 on its upper half and 1 on its lower half). The lower half of the last domino already shows 2 dots. So the hidden upper half must supply 12 − 7 − 2 = 3 dots.

Cross-check: with 3 dots behind the question mark, the last domino totals 3 + 2 = 5, and the final pair totals 7 + 5 = 12 — continuing the 3, 6, 9, 12 pattern exactly and confirming the count.