LR04 — Alphabet and Number Series

🧠 AFCAT Reasoning · LR04 ✈️ AFCAT Level

📌 AFCAT Pattern (LR04): Series is one of the most consistent AFCAT question types — 3–5 questions per paper. You must find the next term or the missing term in a sequence of numbers, letters, or both. The pattern is always logical and consistent. Identify the rule from the first 3–4 terms before answering.

PART A — NUMBER SERIES

1. Key Number Series Patterns

Pattern Type	How to Spot	Example	Next
Arithmetic (AP)	Difference is constant	3, 7, 11, 15, 19	23 (+4 each time)
Geometric (GP)	Ratio is constant	2, 6, 18, 54	162 (×3 each)
Squares	Terms are n²	1, 4, 9, 16, 25	36 (6²)
Cubes	Terms are n³	1, 8, 27, 64	125 (5³)
Fibonacci	Each = sum of two before	1, 1, 2, 3, 5, 8	13
Alternating	Two interleaved series	2, 5, 4, 10, 8, 20, 16	40 (×2 alternately)
Difference of differences	Second-level differences constant	2, 3, 5, 8, 12, 17	23 (diff: 1,2,3,4,5,6)
Prime series	Consecutive primes	2, 3, 5, 7, 11, 13	17
Mixed operations	Alternating ×, +, −	4, 8, 11, 22, 25, 50	53 (×2,+3 alternate)

📌 AFCAT Speed Tip: Always calculate the difference between consecutive terms first. If differences are equal = AP. If differences are increasing/decreasing linearly = second-order series. If ratio is constant = GP. Do this check in under 10 seconds.

2. Worked Examples — Number Series

Example 1: Find missing term

Series: 5, 10, 20, 40, ?, 160
Differences: ×2, ×2, ×2, ?, ×2 → clearly GP with ratio 2
Missing: 40×2 = 80

Example 2: Tricky mixed series

Series: 1, 2, 4, 7, 11, 16, ?
Differences: +1, +2, +3, +4, +5, +6
Next: 16+6 = 22

Example 3: Alternating series

Series: 3, 6, 5, 10, 7, 14, 9, ?
Series 1 (odd positions): 3, 5, 7, 9 → +2 each
Series 2 (even positions): 6, 10, 14, ? → +4 each → 14+4=18

PART B — LETTER SERIES

3. Letter Series Patterns

Common Letter Patterns

Consecutive forward: A,B,C,D → E
Skip alternate: A,C,E,G → I (every other letter)
Reverse: Z,Y,X,W → V
Fixed shift: B,E,H,K,N → Q (+3 each)
Vowel series: A,E,I,O → U

Alphanumeric Series

26Z, 25Y, 24X, 23W → 22V
A1, C3, E5, G7 → I9 (letter+2, number+2)
Z1, X3, V5, T7 → R9 (letter−2, number+2)
A3, D6, G9, J12 → M15 (letter+3, number+3)

📝 AFCAT PYQSeries — AFCAT Pattern

Q1. Find next: 2, 5, 10, 17, 26, ? (AFCAT I 2025)

(a) 35
(b) 36
(c) 37
(d) 38

Answer: (c) 37
Differences: 5−2=3; 10−5=5; 17−10=7; 26−17=9; next difference=11. 26+11=37. Second-level pattern: differences are 3,5,7,9,11 (odd numbers increasing). Alternatively: terms = n²+1 → 1+1=2, 4+1=5, 9+1=10, 16+1=17, 25+1=26, 36+1=37.

Q2. Find the missing term: B, E, H, K, ?, Q (AFCAT II 2024)

(a) M
(b) N
(c) L
(d) O

Answer: (b) N
B(2), E(5), H(8), K(11), ?(14), Q(17). Each letter advances by +3 positions. K(11)+3=N(14)=N. Verify: N(14)+3=Q(17) ✓. Then Q is correct as the term after N.

Q3. Find next: 3, 6, 5, 10, 7, 14, 9, ? (AFCAT I 2024)

(a) 18
(b) 16
(c) 20
(d) 12

Answer: (a) 18
Two interleaved series: Odd positions: 3,5,7,9 (+2 each). Even positions: 6,10,14,? (+4 each). 14+4=18. This is a classic alternating series. Always separate odd and even position terms when the series looks irregular.

Q4. Find next: Z, X, V, T, R, ? (AFCAT II 2023)

(a) P
(b) Q
(c) S
(d) O

Answer: (a) P
Z(26), X(24), V(22), T(20), R(18) — each decreases by 2. R(18)−2=P(16)=P. This is a reverse skip-one series (every other letter in reverse). Note: the series skips odd-positioned letters (Y,W,U,S,Q) and uses even-positioned ones in reverse.

⚡ Quick Reference — LR04

Identify Pattern — Steps

1. Calculate differences between terms
2. If equal → AP (arithmetic)
3. If ratio equal → GP (geometric)
4. If differences increase by same → 2nd order
5. Check for alternating sub-series

Squares & Cubes

Squares: 1,4,9,16,25,36,49,64,81,100,121,144
Cubes: 1,8,27,64,125,216,343
n²+1 or n²−1 patterns are common
n²+n patterns also appear

Letter Shift Cheatsheet

+3: B,E,H,K,N,Q,T,W,Z
+4: A,E,I,M,Q,U,Y
+5: A,F,K,P,U,Z
Always convert to numbers to verify

📝 AFCAT Mock Tests 🎯 Reasoning Quiz ✈️ Telegram