Number Sequence Analysis: Finding the Outlier in 21, 29, 12, 27, 23

To solve this problem, we'll investigate which given number does not fit with the others based on the concept of prime numbers.

• First, identify which numbers are prime:
• $21$ is not prime (divisible by $3$).
• $29$ is prime (divisors are 1 and 29).
• $12$ is not prime (divisible by $2, 3,$ and $4$).
• $27$ is not prime (divisible by $3$).
• $23$ is prime (divisors are 1 and 23).

Among the given numbers, $29$ and $23$ are prime, making 21, 12, and 27 non-prime. However, upon reviewing the numbers again, reconsider the common trivial property: whether they are even or odd numbers.

Now examine a simpler property:

• $21$: Odd
• $29$: Odd
• $12$: Even
• $27$: Odd
• $23$: Odd

With this assessment, $12$ is the only even number, thereby making it the odd one out.

Therefore, the solution to the problem is $12$.