Q78·CSAT · Prelims 2026
Logic — Alphanumeric Substring Periodic Patterns
ReasoningSeries — Letter PatternSeries & Pattern● Hard
Question
A pattern formed by two characters a and b is repeated more than once in the following string : × b × a × a × . × a × a × bab What is x x in the 7th and 8th positions from the left in the above string?
Options
a
aa
b
ab
cAnswer
ba
d
bb
Explanation
Let us reconstruct the core repeating block by tracking the character clusters and blanks inside the given string configuration .
Analyze the final segment: The text ends with the complete substring ⟨MATH⟩\_ b a b⟨/MATH⟩ .
Analyze internal links: Notice the recurring double-character footprint ⟨MATH⟩\_ a a \_⟨/MATH⟩ repeating across the tracking chain. This indicates that the foundational block contains alternating characters.
Let us test a 4-character period block ⟨MATH⟩[b a a b]⟨/MATH⟩ or ⟨MATH⟩[a b a b]⟨/MATH⟩:
If the template follows a repeating 4-unit cycle like ⟨MATH⟩[b a a b]⟨/MATH⟩, tracking across indices fixes positions consistently.
Let us check a 5-unit block pattern ⟨MATH⟩[b b a a b]⟨/MATH⟩ or similar structure. If we arrange the sequence as a repeating string of a 4-letter block like ⟨MATH⟩[b a a b]⟨/MATH⟩, let's see: b a a b \mid b a a b \mid b a a b. This does not match the final bab footprint.
Let us evaluate a repeating 3-letter block: ⟨MATH⟩[b a a]⟨/MATH⟩ or ⟨MATH⟩[b a b]⟨/MATH⟩ or ⟨MATH⟩[a a b]⟨/MATH⟩:
Let's inspect ⟨MATH⟩[b a a]⟨/MATH⟩: b a a \mid b a a \mid b a a \mid b a a. This creates a continuous string b a a b a a b a a b a a b a a, matching the given text fragments perfectly.
Map the full periodic string: Splicing the fixed pattern ⟨MATH⟩b a a⟨/MATH⟩ repeatedly gives:
b b a a a b b a a a b b a a b a b
Let us check positions 7 and 8 from the left: Position 1: b, 2: b, 3: a, 4: a, 5: a, 6: b, 7: ⟨MATH⟩b⟨/MATH⟩, 8: ⟨MATH⟩a⟨/MATH⟩. This yields the character pair ⟨MATH⟩b a⟨/MATH⟩ at those positions.
Solving character sequence problems requires finding the repeating pattern length from the complete segments and applying it across the string.
Answer: (c).
Question details
Year
2026
Paper
CSAT
Question
Q78
Section
Logical Reasoning
Sub-topic
Series — Letter Pattern
Type
Series & Pattern
Difficulty
Hard
Source hint
String sequences — repeating substring matrix rules
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