Series (Number & Letter)
🔒 Log in to trackAlphanumeric series
🔒 Log in to trackTerms mix a letter and a number ('A3, D6, G9, J12, ?'). The letter track and the number track are independent series. Solve each separately, then bolt the answer together: letter from track 1, number from track 2.
Common track rules: letters step ±2/±3 (or reverse), numbers run an AP (step 3), a ×2 GP, or squares (1, 4, 9, 16). Nothing couples the tracks — if you ever need the number to compute the letter, you have misread the question.
Detailed notes
What an alphanumeric series is
'Alpha' means letters and 'numeric' means numbers. Each term mixes both, like A3, D6, G9. Think of seats in a train: S1, S2, S3 is the coach series and 12, 13, 14 is the seat series. The two parts usually move by their own separate rules.
Rule 1: two independent tracks
Split every term into its letter part and its number part. Solve each as a separate series, then join the answers.
Example: B2, D4, F8, H16, ? → letters B, D, F, H (+2) → J; numbers 2, 4, 8, 16 (×2) → 32. Answer J32.
The letter can come before or after the number: 3Z, 7W, 11T → numbers +4, letters −3 → 15Q.
Rule 2: number linked to the letter
Sometimes the number is not a separate series. It is the position of the letter, or something made from it:
- C3, F6, I9 → number = position of the letter.
- B4, D16, F36 → number = (position)².
- Z26, X24, V22 → number = position, letters going backwards.
Check this link first. If it holds for every term, you only need the next letter; the number follows.
Rule 3: clusters with numbers
Terms can have two letters and a number: AB2, DE4, GH8. Each letter slot is its own series (A, D, G and B, E, H, both +3) and the number doubles → JK16. Or the number comes first: 2CZ, 4EX, 6GV → numbers +2, first letters +2, last letters −2.
Rule 4: missing term in the middle
When the '?' sits in the middle, find each track from the visible terms and fill the gap. Always check the term after the gap.
Example: D5, G10, ?, M20, P25 → letters +3 → J; numbers +5 → 15. Answer J15.
Method
- Draw two (or three) columns: letters and numbers.
- Convert letters to positions (EJOTY anchors) and find the step.
- Find the number rule: +k, ×k, squares, cubes, or 'same as the letter position'.
- Build the answer in the same format as the terms (letter first or number first).
- Before marking, compare options that differ in only one character — examiners pair the right letter with a wrong number and the right number with a wrong letter.
Common traps
- An option with the correct letter but a number from a wrong rule (J30 instead of J32).
- An option in the wrong order (32J instead of J32).
- Moving the letter by the number's step, or the number by the letter's step.
Quick revision
| Series | Letter rule | Number rule | Next |
|---|---|---|---|
| B2, D4, F8, H16 | +2 | ×2 | J32 |
| C3, F6, I9, L12 | +3 | = position | O15 |
| B4, D16, F36, H64 | +2 | position² | J100 |
| AB2, DE4, GH8, JK16 | +3 each slot | ×2 | MN32 |
| Z1, W8, T27, Q64 | −3 | cubes | N125 |
- Split, solve, join.
- Test 'number = letter position' first.
- Keep the answer in the same format as the terms.
Types of questions asked
Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.
Type 1: Letter + number terms with two independent tracksvery common3 practice Q
Each term is one letter and one number (B2, D4 ...), and the number does not match the letter position.
- Write the letters in one column and the numbers in another.
- Solve the letter track (positions, step) and the number track (+k, ×k, squares) separately.
- Join the answers in the same format.
Why it works: the two parts are two separate series written side by side.
Example: K5, M10, O15, Q20, ?
Letters K, M, O, Q (+2) → S. Numbers 5, 10, 15, 20 (+5) → 25. Answer S25.
Type 2: Number linked to the letter's positioncommon3 practice Q
The number equals the letter's position, its square, or its reverse position (C3, F6 / B4, D16).
- Check if number = position (or position², or 27 − position) for every term.
- Find the next letter.
- Compute its number by the same link.
Why it works: once the link is confirmed, the whole question reduces to one letter series.
Example: E5, H8, K11, ?
Numbers equal positions (E=5, H=8, K=11). Letters +3 → N = 14. Answer N14.
Type 3: Clusters with two or more letters and a numbercommon3 practice Q
Terms like AB2, DE4 or 2CZ, 4EX — more than one letter slot plus a number.
- Make one column per slot (each letter place, the number).
- Solve every column separately.
- Assemble the next term in the same order.
Why it works: every slot is independent; three small series are easier than one big cluster.
Example: AZ1, BY2, CX3, ?
First letters A, B, C (+1) → D; second letters Z, Y, X (−1) → W; numbers +1 → 4. Answer DW4.
Type 4: Missing alphanumeric term in the middlecommon3 practice Q
The '?' is in the middle of an alphanumeric series.
- Find each track from the visible terms on both sides.
- Fill the letter and the number of the gap.
- Check the term after the gap.
Why it works: each track must pass through the gap smoothly.
Example: B3, E6, ?, K12, N15
Letters B, E, _, K, N (+3) → H. Numbers 3, 6, _, 12, 15 (+3) → 9. Answer H9.
Shortcut tricks
⚡ Two columns, two answers, one bolt
Make a letter column and a number column. Solve the letter column (step row), solve the number column, then write them side by side in the original order.
Example: Q. A3, D6, G9, J12, ?
Sol. Letters: A, D, G, J (+3) → M. Numbers: 3, 6, 9, 12 (+3) → 15. Answer M15.
Tracks are independent — an error stays inside one column.
Where students lose marks
Reading 'A3' as one base-30 code instead of two independent tracks.
Multiplying numbers when the track is a plain AP (check +3 vs ×2 quickly).
Mixing up the bolt order (writing 15M) — always mirror the question's order.
Practice sets — 14 questions
Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.
Topic test · 10 questions
Suggested time 6 min · wrong answers go to your mistake notebook automatically.