Choose the odd number: Among 78, 48, 72, and 54, exactly one number has 13 as a prime factor. Identify that distinct number.

Difficulty: Medium

Correct Answer: 78

Explanation:


Introduction / Context:
Several classification items rely on a specific hidden factor. Here, we test for the presence of the prime 13 in the factorization.


Given Data / Assumptions:

  • Numbers: 78, 48, 72, 54.
  • Check prime factor 13.


Concept / Approach:
Factor each number or check divisibility by 13. Because 13 * 6 = 78, this quickly singles out the outlier.


Step-by-Step Solution:

78 = 13 * 6 → contains factor 13.48 = 2^4 * 3 → no factor 13.72 = 2^3 * 3^2 → no factor 13.54 = 2 * 3^3 → no factor 13.


Verification / Alternative check:
Dividing each by 13: only 78/13 = 6 is integer; others are not.


Why Other Options Are Wrong:
48, 72, and 54 share small prime factors 2 and 3 but lack 13.


Common Pitfalls:
Using only parity or sum-of-digits rules; those do not detect the presence of 13.


Final Answer:
78 is the only number with prime factor 13 and is the odd number.

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