3.5 Alphabetical & Alphanumeric Series

Why This Topic Is Nearly Free Marks

The official Paper-I syllabus lists Alphabetical Series as its first Logical Reasoning bullet, distinct from Number Series. Candidates who prepare only number series lose these marks unnecessarily. The entire topic reduces to one idea: treat every letter as its position number, do arithmetic, then convert back.

The Position Table

The EJOTY device

Rather than counting from A every time, memorise five anchors:

$E = 5, \quad J = 10, \quad O = 15, \quad T = 20, \quad Y = 25$

Any letter is then found by stepping a short distance from the nearest anchor. To locate R: it is two after P, and P is one after O at 15, so P is 16, Q is 17, R is 18. Two seconds, no counting from A.

The opposite-letter rule

The mirror of a letter at forward position $n$ sits at position $27 - n$:

$\text{Forward position} + \text{Backward position} = 27$

So A pairs with Z, B with Y, C with X, and the central pair is M with N. Memorise the shortcut string AZ BY CX DW EV FU GT HS IR JQ KP LO MN and mirror questions become instant.

Type 1: Continuous Pattern Series

Find the next term in C, F, I, L, ?

Positions are 3, 6, 9, 12 — a common difference of 3. The next position is 15, which is O.

Now A, C, F, J, ?

Positions 1, 3, 6, 10 with differences 2, 3, 4. The next difference is 5, giving position 15, again O. Always write the positions in a row before looking for the rule; patterns that are invisible in letters are obvious in numbers.

Type 2: Alternating and Two-Line Series

Some series interleave two independent sequences. In A, Z, C, X, E, V, ?, take alternate terms:

• Odd positions: A (1), C (3), E (5) — increasing by 2, so next is G (7).
• Even positions: Z (26), X (24), V (22) — decreasing by 2.

The question asks for the seventh term, which belongs to the odd sequence: G.

Type 3: Letter Groups

In ABD, EFH, IJL, ? examine each group's internal structure. Within ABD the positions are 1, 2, 4 — a gap pattern of +1 then +2. Groups start at 1, 5, 9, so the next starts at 13 (M), giving M, N and then +2 to P: MNP.

Type 4: Alphanumeric and Mixed Series

These interleave letters, digits and sometimes symbols, then ask a counting question. Consider the arrangement

P 4 R $ 7 M K 2 % B 9 D T 5 @ Q

Typical questions:

• How many letters are immediately preceded by a digit? Scan left to right and check each letter's predecessor: R is preceded by 4 (yes), M by 7 (yes), B by % (no), D by 9 (yes), Q by @ (no). Answer: 3.
• Which element is fourth to the right of the seventh element from the left? The seventh element is K; four to its right is 9. A faster route: the element is at position 7 + 4 = 11, which is 9.
• How many symbols are immediately followed by a letter? $ is followed by 7 (no), % by B (yes), @ by Q (yes). Answer: 2.

The discipline that prevents errors: number the positions above the sequence before answering anything. Almost every mistake in this topic is a miscount, not a misunderstanding.

Type 5: Position-Based Questions

These use the alphabet itself rather than a given string.

Which letter is 8th to the left of the 20th letter from the left? The 20th letter is T at position 20; eight to the left is position 12, which is L. In general:

$\text{Required position} = 20 - 8 = 12$

Which letter is 5th to the right of the 12th letter from the right? Counting from the right, the 12th letter is at forward position $27 - 12 = 15$, which is O. Five to the right gives position 20, which is T.

The general rule for combined counts from opposite ends is worth writing down once:

$\text{If } m \text{th from left and } n \text{th from right in a row of } N, \text{ then } m + n = N + 1$

Type 6: Rearrangement Questions

Some items ask which letter would occupy a given position if the alphabet were reversed, or if the first half were interchanged with the second half.

If the alphabet is written in reverse, which letter is 9th from the left? Reversing puts Z first, so the 9th is at forward position $27 - 9 = 18$, which is R.

If the first thirteen letters are reversed among themselves, which letter is 5th? The first half A to M reversed becomes M, L, K, J, I, so the fifth is I.

Worked Example Set

Exam Technique

Work the positions, not the letters. Keep a mental EJOTY anchor. Because the CIL CBT applies no negative marking, if a series resists analysis within about forty seconds, choose the option nearest your partial pattern, flag it, and move on — an unattempted item scores zero, whereas a considered guess among four options carries a one-in-four chance of a mark.