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

Concept

In an alternating (interleaved) number series, the terms at the odd positions form one hidden sub-series and the terms at the even positions form another. Each sub-series follows its own simple rule, so you must separate the two before looking for any pattern.

Application

Split the series 20, 32, 47, 57, 74, ? by position and work out each hidden sub-series' own rule:

1. Separate the terms: odd positions (1, 3, 5) give 20, 47, 74; even positions (2, 4, 6) give 32, 57, ?.

2. Odd-position gaps: 47 − 20 = 27 and 74 − 47 = 27 — a constant +27.

3. Even-position first gap: 57 − 32 = 25; the gaps in this sub-series shrink by 2 at each step, so the next gap is 25 − 2 = 23.

4. The missing term is the 6th (even-position) term, so add the derived gap to the previous even-position term: 57 + 23 = 80.

Cross-check

Read the complete series back: 20, 32, 47, 57, 74, 80. Odd terms 20 → 47 → 74 advance by a steady +27, and even terms 32 → 57 → 80 advance by +25 then +23 (each gap 2 less than the last). Both sub-series stay consistent, so the answer is 80.