In the series 1, 3, 6, 11, 18, ?, which number should replace the question mark so that the pattern continues correctly?

Introduction / Context:
This number series grows moderately, suggesting an additive pattern with changing differences rather than an aggressive exponential or purely multiplicative rule. Many reasoning questions use differences based on prime numbers or similar simple sequences. Recognising the structure in these differences is the key here.

Given Data / Assumptions:
- The series is: 1, 3, 6, 11, 18, ?- All terms are positive integers.- The pattern appears to involve increasing additive steps.

Concept / Approach:
We first calculate the differences between consecutive terms. If these differences form a recognisable pattern, such as successive prime numbers or another classic sequence, we extend that pattern to find the next difference and thus the missing term. This is a very standard and efficient technique for this style of question.

Step-by-Step Solution:
- Compute differences: 3 - 1 = 2, 6 - 3 = 3, 11 - 6 = 5, 18 - 11 = 7.- The difference sequence is: 2, 3, 5, 7.- These numbers are the first four prime numbers in order.- The next difference should naturally be the next prime, which is 11.- Add this to the last known term: 18 + 11 = 29.- Therefore, the missing term is 29.

Verification / Alternative check:
- Rebuild the series from 1 using prime differences: 1 + 2 = 3, 3 + 3 = 6, 6 + 5 = 11, 11 + 7 = 18, 18 + 11 = 29.- Every step fits the pattern of adding consecutive primes: 2, 3, 5, 7, 11.- This confirms that 29 is the only correct value for the missing term.

Why Other Options Are Wrong:
- 27, 28, and 31 cannot be obtained by adding the next prime number 11 to 18.- Using any of these alternatives would break the clean prime difference pattern and lead to inconsistency.- Only 29 preserves the structure of consecutive prime increments.

Common Pitfalls:
Students may attempt to fit an arbitrary quadratic or multiplicative rule instead of checking simple difference patterns. Others may overlook that 2, 3, 5, 7 are prime numbers and miss the opportunity to extend the prime sequence. Always inspect the difference sequence for familiar number families such as primes, squares, or cubes.

Final Answer:
The correct number to complete the series is 29, so the correct option is 29.