Series (Number & Letter)
🔒 Log in to trackSeries questions ask for the next or missing term of a number or letter sequence, or the term that spoils a given sequence. CGL asks 4-5 per Tier 1 shift. The universal tool is the difference ladder; the recurring families are AP, ×k±r, squares/cubes, interleaved pairs and stepped letter tracks.
One page per subtopic: detailed notes, every question type, formulas, tricks and practice sets.
Every formula on one printable page, grouped by subtopic.
4 exam-level questions worked step by step.
95 questions — untimed practice or a timed test with analysis.
Track record in the exam
Questions per shift in recent SSC CGL papers.
Test difficulty mix (95 questions)
Question patterns exams keep repeating
Taken from previous-year papers. If a pattern is marked "very common", expect to see it in your exam.
Growing-difference number series
very commonTerms rise slowly and the gaps between them rise (or fall) by a fixed amount.
How to solve: Write the difference row and continue it. If the first row is not regular, take a second row. Add the next difference to the last term.
Example: 11, 13, 17, 23, 31, ?
Differences 2, 4, 6, 8 → next 10. 31 + 10 = 41.
Multiply-and-add series (×k ± r)
very commonTerms roughly double or triple each time.
How to solve: Divide a term by the one before to guess k, then find the small constant left over. Confirm the same k and r on every step before applying it.
Example: 4, 10, 28, 82, ?
Rule ×3 − 2. 82 × 3 − 2 = 244.
Squares, cubes and n(n+1) series
commonTerms sit on or just beside perfect squares or cubes (3, 8, 15, 24 or 2, 9, 28, 65).
How to solve: Compare each term with the nearest square or cube and find the fixed adjustment. Use the next n. Knowing squares to 30 and cubes to 15 makes this instant.
Example: 2, 9, 28, 65, ?
Terms are n³ + 1. 5³ + 1 = 126.
Mixed-operation and alternate series
commonGrowth is uneven (×1+1, ×2+2 ...) or the series goes up and down.
How to solve: First try splitting odd and even places into two series. If that fails, list the operation used at each step and continue the operation pattern.
Example: 2, 3, 8, 27, 112, ?
×1+1, ×2+2, ×3+3, ×4+4 → next ×5+5: 112 × 5 + 5 = 565.
Missing term in the middle
very commonThe '?' is between given terms instead of at the end.
How to solve: Find the rule from the visible terms on both sides. Fill the gap from the left neighbour and check that the right neighbour follows from your answer.
Example: 5, 8, 14, ?, 50, 98
Rule ×2 − 2. ? = 14 × 2 − 2 = 26; check 26 × 2 − 2 = 50 ✓.
Letter and letter-cluster series
very commonSingle letters or groups of 2-4 letters follow a step (BDF, GIK, LNP ...).
How to solve: Convert letters to positions with the EJOTY anchors. For clusters, solve each slot as its own series. Remember wrap-around after Z.
Example: AZ, CX, EV, GT, ?
First letters +2 → I; second letters −2 → R. Answer IR.
Alphanumeric series
commonTerms mix letters and numbers (A3, D6, G9 or 2CZ, 4EX).
How to solve: Split into a letter column and a number column. Check first whether the number equals the letter's position. Solve each column and join the answer in the same format.
Example: B2, D4, F8, H16, ?
Letters +2 → J; numbers ×2 → 32. Answer J32.
Wrong number in the series
commonThe series has no '?'; the question asks which term is wrong, and the options are terms of the series.
How to solve: Find the rule from the first few terms and rebuild the series from the start. The one term that does not match is the answer. A wrong term spoils both gaps around it.
Example: 2, 6, 18, 54, 164, 486
Rule ×3 gives 162, not 164 → 164 is wrong.
Fill the blanks in a repeating letter series
very commonA line of small letters with blanks, e.g. 'a _ c a b _ a b c _ b c'; options are letter groups.
How to solve: Count the places and guess the block size. Write the blocks one under another and fill each blank from its column. Watch for growing, mirror and rotating blocks.
Example: a _ c a b _ a b c _ b c
Block abc repeats. Blanks: b, c, a → bca.