In the alphanumeric series 3F, 6G, 11I, 18L, ?, each term combines a number and a letter. Which of the following options correctly completes the pattern?

Introduction / Context:
This problem involves an alphanumeric sequence where each term consists of a number followed by a letter. To solve it, we must identify independent patterns in the number part and the letter part. Such questions test the ability to handle simultaneous logical rules and to keep numerical and alphabetical progressions in mind at the same time.

Given Data / Assumptions:

• The series is: 3F, 6G, 11I, 18L, ?
• Each term has a numeric component and a single uppercase letter.
• Exactly one term is missing at the end and must be chosen from the options.
• The numeric sequence and the letter sequence both follow consistent rules.

Concept / Approach:
We treat the numbers and letters separately. For numbers, computing differences often reveals the pattern. For letters, we convert them to alphabetical positions (A = 1, B = 2, …, Z = 26) and look at how these positions progress. Often, the number rule and the letter rule are independent but parallel, using similar ideas like increasing differences or step sizes. Once both patterns are understood, we combine them to form the missing term.

Step-by-Step Solution:
Step 1: Analyse the numeric part: 3, 6, 11, 18. Differences: 6 - 3 = 3, 11 - 6 = 5, 18 - 11 = 7. Step 2: Observe that the differences form the series 3, 5, 7, which are consecutive odd numbers. Step 3: The next difference should be the next odd number, 9. Step 4: So the next number is 18 + 9 = 27. Step 5: Now analyse the letter part: F, G, I, L. Convert to positions: F = 6, G = 7, I = 9, L = 12. Step 6: Differences between these positions: 7 - 6 = 1, 9 - 7 = 2, 12 - 9 = 3. Step 7: The differences are 1, 2, 3 so the next increment should be 4. Step 8: Thus the next letter position is 12 + 4 = 16, which corresponds to P. Step 9: Combining the numeric and letter patterns, the missing term is 27P.

Verification / Alternative check:
We can reconstruct each term using these rules. Starting from 3F, add 3 to get 6 and move from F(6) by +1 step to G(7). Then add 5 to get 11 and move +2 letters to I(9). Then add 7 to get 18 and move +3 letters to L(12). The next step is naturally +9 in numbers and +4 in letters, yielding 27P. The structure is fully consistent and symmetrical, which strongly confirms the solution.

Why Other Options Are Wrong:
Terms like 27Q, 25N or 21O break at least one of the patterns. For example, 25N would require the numeric sequence to skip from 18 to 25 (difference 7 instead of 9) and the letter sequence to jump to N(14), which does not follow the +4 increment from L(12). Likewise, 27Q uses the correct number 27 but the letter Q(17) is one step ahead of the correct position 16 (P).

Common Pitfalls:
A frequent error is to treat the pair as a single unit and look for very complex transformations, rather than splitting the analysis into numeric and alphabetic parts. Another pitfall is to spot only the number pattern and then guess a random letter, which might coincidentally look plausible but will not survive a careful position based check.

Final Answer:
The correct term that continues both the number pattern of increasing odd differences and the letter pattern of increasing step sizes is 27P.