UPSC Prelims 2025 CSAT, question 60

Solution.

Answer: (c) 29

1. The differences between consecutive terms of $1, 3, 6, 11, 18, X, 42$ start with $3 - 1 = 2$, $6 - 3 = 3$, $11 - 6 = 5$, and $18 - 11 = 7$, which are consecutive prime numbers.
2. Adding the next two primes $11$ and $13$ gives $X = 18 + 11 = 29$ and $29 + 13 = 42$.