UPSC CSE 2025 · PYQ · Number Series · medium
What is X in the sequence 1, 3, 6, 11, 18, X, 42?
- A.26
- B.27
- C.29✓ Correct
- D.30
Examine differences: 3−1=2, 6−3=3, 11−6=5, 18−11=7, next difference should follow the pattern of primes: 2, 3, 5, 7, 11, 13. So next difference = 11, giving X = 18 + 11 = 29. Verify: 42 − 29 = 13 ✓ (next prime). The differences are consecutive primes 2, 3, 5, 7, 11, 13. So X = 29. Answer is (c) 29.
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