Find the Outlier in Number Sequence: 21,11,77,88,55

We will analyze the characteristics of each given number: $21, 11, 77, 88, 55$. Typically, numbers are compared based on their properties, such as digit repetition within two-digit numbers.

• $11, 77, 88,$ and $55$ all have repeating digits: $11 = 1-1$, $77 = 7-7$, $88 = 8-8$, $55 = 5-5$.
• $21$ does not have repeating digits; it is composed of two distinct digits: $2$ and $1$.

Based on this observation, $21$ is the number among the list that does not follow the pattern of having repeated digits.

Therefore, the number $21$ is the outlier.

To conclude, the solution to the problem is $21$.