Consider the following sequence of numbers:
5 1 4 7 3 9 8 5 7 2 6 3 1 5 8 6 3 8 5 2 2 4 3 4 9 6
How many odd numbers are followed by the odd number in the above sequence?
Answer & explanation
Answer: (b) 6
Scan the 26 digits once and mark every place where an odd digit is immediately followed by another odd digit. There are six such places: 5-1, 7-3, 3-9, 5-7, 3-1 and 1-5.
- Pairs in order: 5-1 ✓, 1-4, 4-7, 7-3 ✓, 3-9 ✓, 9-8, 8-5, 5-7 ✓, 7-2, 2-6, 6-3, 3-1 ✓, 1-5 ✓, 5-8 …
- After 5-8 the odd digits are 3, 5, 3 and 9, and each is followed by an even digit (8, 2, 4, 6).
- Total odd-odd pairs = 6.
Remember · For adjacency counts, scan pairs left to right in one pass and tick each hit; do not restart counting.
Question and answer: UPSC's official GS Paper II (2020, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). ·