Question: An anthropologist studies 7 distinct cultural practices, each with 2 possible variations. How many ways can they select 3 practices and assign a variation to each, ensuring all selected practices have different variations? - NBX Soluciones

Understanding the Context

At first glance, the task appears simple: pick 3 out of 7 cultural practices, then assign one of two variations per selected practice. But ensuring that all three chosen practices carry distinct variations introduces a subtle layer of combinatorics — one that reflects how real-world cultures often embed diversity within defined frameworks.

The core challenge lies in aligning selection with variation constraints: each selected practice must receive a unique variation, meaning no two practices can share the same setting. This prevents overlap and echoes how cultural variation is rarely arbitrary — it’s tied closely to context.

Step-by-Step Logic: Selection and Assignment

To answer the question thoroughly, break it into two parallel actions:

1. Selecting 3 practices from 7
2. Assigning a unique variation to each selected practice

Key Insights

Step 1: Choosing 3 Practices from 7

Start with the combination of selection — choosing any 3 out of 7 cultural practices. This is a foundational combinatorial calculation:
[ \binom{7}{3} = \frac{7!}{3!(7-3)!} = 35 ]
So, there are 35 unique sets of 3 practices that can be chosen.

Step 2: Assigning Variations with Unique Pairings

Each selected practice has 2 possible variations. The constraint that all selected practices must have different variations means no duplicates among the three assigned variants. This turns the assignment into a permutation challenge: how many ways can 3 distinct variations be assigned across the 3 practices?

With 2 options per practice, and requiring variety across all three, we’re effectively choosing 3 distinct values from a pool of 2 — but only if they differ per practice. The number of valid variation assignments requires deeper inspection.

Final Thoughts

Because each practice has two distinct options, and the centered rule mandates no repetition among the three, this becomes a distribution problem with uniqueness constraints. For three selected practices and two available variations, it’s mathematically impossible to assign a unique variation to each without repeating at least one — unless some variation appears only once and others appear multiple times.

But wait — the requirement is that all selected practices have different variations. Since each source practice has only two options, and we need three distinct ones, this constraint cannot be fully satisfied if all must differ. The key insight: it is impossible to assign three distinct variations from only two choices — at least one variation will repeat.

However, interpreting the question carefully, “