Find X in sequence 1,3,6,11,18,X,42

Write down the sequence and calculate the first-order differences between consecutive terms: 3 - 1 = 2 6 - 3 = 3 11 - 6 = 5 18 - 11 = 7 The differences are 2, 3, 5, 7. This is the sequence of consecutive prime numbers. The next prime number after 7 is 11. Therefore, X = 18 + 11 = 29. To verify, the prime after 11 is 13. Adding this to our X yields 29 + 13 = 42, which perfectly matches the final term in the sequence.

Answer: (c).