What should come in place of question mark (?) in the following number series?…

Concept: This is a skip-position (interleaved) number series — the terms split into two independent sub-series, one running through the 1st, 3rd, 5th... positions and the other through the 2nd, 4th, 6th... positions. Each sub-series follows a “double and add a constant” rule: a term equals twice the term two places before it, plus a fixed constant specific to that sub-series.

Application:

1. Split the series 3, 12, 7, 26, 15, ? by position: the odd positions (1st, 3rd, 5th) give 3, 7, 15, and the even positions (2nd, 4th, 6th) give 12, 26, ?.

2. Confirm the odd-position rule using the known odd-position terms: 7 = 3 × 2 + 1, and 15 = 7 × 2 + 1 — so each odd-position term equals twice the previous odd-position term plus 1.

3. Confirm the even-position rule using the known even-position terms: 26 = 12 × 2 + 2 — so each even-position term equals twice the previous even-position term plus 2.

4. The missing term is the 6th term, which is an even position, so apply the even-position rule to the 4th term (26): 26 × 2 + 2 = 54.

Cross-check: The value 54 belongs strictly to the even-position chain (12 → 26 → 54); the odd-position chain (3 → 7 → 15 → ...) would only produce the 7th term, not the 6th, confirming the two chains were not mixed.