Compute the arithmetic mean (average) of the six numbers: 8, 9, 12, 13, 15, and 9.

Introduction / Context:
The arithmetic mean of a finite set of numbers is the sum of the numbers divided by how many numbers there are. This is a foundational concept used across aptitude and data analysis questions.

Given Data / Assumptions:

• Numbers: 8, 9, 12, 13, 15, 9
• Count of numbers: 6

Concept / Approach:
Average = (sum of all values) / (number of values). Maintain correct order of operations and careful addition to avoid arithmetic slips.

Step-by-Step Solution:

Verification / Alternative check:
Cross-check by direct addition in a single pass: 8 + 9 = 17; +12 = 29; +13 = 42; +15 = 57; +9 = 66; 66 / 6 = 11. Same result confirms correctness.

Why Other Options Are Wrong:

• 6: This is the count, not the computed mean.
• 16: Larger than all but one term; average cannot exceed the maximum value in a simple set without weights.
• 18: Exceeds the maximum; impossible for an unweighted mean of these values.
• None of these: A matching correct value exists (11).

Common Pitfalls:
Miscounting the number of terms or making a slip in addition can lead to off-by-one errors in the average.

Final Answer:
11