Detecting the pattern in a sequence — arithmetic, geometric, and alternating series across numbers, letters, and mixed formats.
📖 ~13 min read🧩 SSC Reasoning
Introduction
Series questions test how quickly you can detect the underlying pattern connecting consecutive terms —
whether the sequence is made of numbers, letters, or an alternating mix of both.
Diagram — Reading the Gaps Between Terms
Always write the gap between each consecutive pair first — the gap pattern (here, +4, +6, +8, +10) reveals the rule (increasing even differences). Next term = 20+10 = 30.
Common Number Series Patterns
Pattern Type
Example
Arithmetic (constant difference)
5, 10, 15, 20...
Geometric (constant ratio)
3, 6, 12, 24...
Increasing difference
2, 6, 12, 20, 30... (differences: 4,6,8,10)
Squares/Cubes series
1, 4, 9, 16, 25... or 1, 8, 27, 64...
Alternating operations
×2, +1, ×2, +1... pattern
Two interleaved series
Odd positions form one series, even positions another
Q. Find the next term: 1, 4, 9, 16, 25, ?
Pattern: n² series (1²,2²,3²,4²,5²)
Next: 6² = 36
Alphabet Series
Assign each letter its position number (A=1, B=2...Z=26), find the numeric pattern, then convert back to letters.
Q. Find the next term: B, D, G, K, ?
Positions: B=2, D=4, G=7, K=11 → gaps: +2, +3, +4
Next gap = +5 → 11+5 = 16 → P
💡 Exam Tip: If a single difference/ratio rule doesn't fit all terms, immediately test two things: (1) whether the gaps themselves form a pattern (2nd-order differences), and (2) whether alternate terms form two separate series — these two checks solve the vast majority of "difficult" series questions.
✅ Practice Focus: Writing out gaps between consecutive terms first · Recognising square/cube series instantly · Alphabet-to-number conversion for letter series · Two-interleaved-series detection.