A series question gives a pattern and asks for the missing or next term. The method is always the same: look at differences first, then ratios, then squares and cubes, then mixed patterns. Never guess from the first two terms alone.
Worked example: 3, 7, 13, 21, 31, ?. Differences are 4, 6, 8, 10 — rising by 2 each time. The next difference is 12, so the next term is 31 + 12 = 43. One pattern, checked across all terms, beats a clever guess from the first pair.