Analyzing the Letter Series Completion Problem

The problem asks us to find the correct combination of letters from the given options that will complete the provided letter series. To solve this type of logical reasoning question, we need to identify the pattern governing the sequence of letters.

The given series with blanks is:

A _ _ BCD_ AB_ _ DAA _ _ CDA _ _ BCD

Let's count the total number of positions in the series, including the blanks. Counting carefully:

A (1) + \_ \_ (2) + BCD (3) + \_ (1) + AB (2) + \_ \_ (2) + DAA (3) + \_ \_ (2) + CDA (3) + \_ \_ (2) + BCD (3) = 1 + 2 + 3 + 1 + 2 + 2 + 3 + 2 + 3 + 2 + 3 = 24 characters.

The total length of the completed series is 24 characters. The number of blanks is 9.

We are given four options, each containing 9 letters to fill the 9 blanks. We need to test each option to see if it creates a discernible pattern in the series.

Let's try inserting the letters from the options into the blanks. The blanks are at positions (1-based index): 2, 3, 7, 10, 11, 15, 16, 20, 21.

Let's examine the third option, which is ABABCBBAB.

Inserting these letters sequentially into the blanks:

• Blank 1 (pos 2) gets 'A'
• Blank 2 (pos 3) gets 'B'
• Blank 3 (pos 7) gets 'A'
• Blank 4 (pos 10) gets 'B'
• Blank 5 (pos 11) gets 'C'
• Blank 6 (pos 15) gets 'B'
• Blank 7 (pos 16) gets 'B'
• Blank 8 (pos 20) gets 'A'
• Blank 9 (pos 21) gets 'B'

Filling the blanks in the original series A _ _ BCD_ AB_ _ DAA _ _ CDA _ _ BCD:

A (A) (B) B C D (A) A B (B) (C) D A A (B) (B) C D A (A) (B) B C D

The completed series is:

A A B B C D A A B B C D A A B B C D A A B B C D

Now, let's look for a pattern in this completed series. The total length is 24. Let's try dividing it into equal blocks. Possible block lengths are divisors of 24: 2, 3, 4, 6, 8, 12, 24.

Let's consider a block length of 6:

• Block 1: AABBCD (characters 1-6)
• Block 2: AABBCD (characters 7-12)
• Block 3: AABBCD (characters 13-18)
• Block 4: AABBCD (characters 19-24)

We can clearly see that the completed series consists of the 6-character block "AABBCD" repeated exactly 4 times.

This demonstrates a consistent and repeating pattern, which is the goal of these types of series completion problems. Let's verify if inserting the letters from option 3 actually produces this sequence and fills the original blanks correctly based on the AABBCD repeating pattern.

As shown in the table, inserting the letters ABABCBBAB from option 3 into the blanks of the original series perfectly aligns with the repeating pattern "AABBCD". Therefore, option 3 correctly completes the series.

Testing other options would not yield such a clear repeating pattern.

Understanding Letter Series Patterns

Letter series questions often involve identifying patterns which could be:

• Simple repetition of a block of letters.
• Progression based on alphabetical order (forward or backward).
• Skipping letters in a sequence.
• Alternating patterns.
• Patterns based on position numbers or mathematical operations (less common for simple letter series).

In this specific problem, the pattern was a simple repetition of a block.

Revision Table: Key Steps for Series Completion

Additional Information on Logical Series

Logical series problems, including letter series, number series, and mixed series, are common in aptitude tests. They assess your ability to identify rules and patterns. Practice with different types of series helps in quickly recognizing the underlying logic. For letter series, understanding basic alphabetical order and common short sequences (like ABCD, PQRS) is helpful. Sometimes, the pattern might involve skipping a fixed number of letters, moving in reverse order, or a combination of simple rules applied sequentially or alternately.