Q. What is X in the sequence 1, 3, 6, 11, 18, X, 42?

Explanation : 

1. List the terms and their differences:
Sequence: 1, 3, 6, 11, 18, X, 42
Differences:
3–1 = 2
6–3 = 3
11–6 = 5
18–11 = 7
X–18 = ?
42–X = ?

2. Observe the differences so far: 2, 3, 5, 7 – these are consecutive primes.

3. The next two primes after 7 are 11 and 13.
So
X – 18 = 11 ⇒ X = 18 + 11 = 29
42 – X = 13 ⇒ 42 – 29 = 13 (checks out)

Thus X = 29.

Answer: (c) 29.