Letter Series Pattern Deconstruction

The series consists of 4-letter blocks: ZYXW, STUV, RQPO, ?, JIHG, CDEF. We need to identify the pattern governing these blocks.

Identify Block Patterns

The blocks follow alternating internal patterns:

• Descending: Letters decrease alphabetically (e.g., ZYXW).
• Ascending: Letters increase alphabetically (e.g., STUV).

The series alternates: Descending (ZYXW), Ascending (STUV), Descending (RQPO), ? (Ascending), Descending (JIHG), Ascending (CDEF).

Analyze First Letter Sequence Differences

Consider the first letter of each block and its alphabetical position (A=1, Z=26):

The differences between consecutive starting letter positions are:

• 26 -> 19: -7
• 19 -> 18: -1
• 18 -> ?: ?
• ? -> 10: ?
• 10 -> 3: -7

This reveals a repeating difference pattern: -7, -1.

Determine Missing Ascending Block

Based on the pattern -7, -1, the next difference after 18 should be -7:

• Starting position of ? = 18 + (-7) = 11

The 11th letter is K. The missing block must start with K.

As established, this position requires an ascending block. The ascending sequence starting with K is KLMN.

Verify KLMN Replacement in Series

The complete series: ZYXW, STUV, RQPO, KLMN, JIHG, CDEF.

• Internal patterns: Descending, Ascending, Descending, Ascending, Descending, Ascending. (Matches)
• First letter sequence: Z(26), S(19), R(18), K(11), J(10), C(3).
• Differences: -7, -1, -7, -1, -7. (Matches)

The cluster KLMN correctly completes the series.