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

1. A. 26
2. B. 27
3. C. 29
4. D. 30

Answer: C

Explanation

Let’s find the differences between consecutive terms in the sequence:
* 3 – 1 = 2
* 6 – 3 = 3
* 11 – 6 = 5
* 18 – 11 = 7
The differences are 2, 3, 5, 7. These are consecutive prime numbers.
Following this pattern, the next difference should be the next prime number after 7, which is 11.
So, X – 18 = 11
X = 18 + 11 = 29.

Let’s check the next difference to confirm the pattern:
The prime number after 11 is 13.
So, 42 – X should be 13.
42 – 29 = 13. This confirms the pattern.

Therefore, X = 29. This question tests pattern recognition in number series, a fundamental skill in logical reasoning.