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

Q: What if multiple numbers could be outliers?

Look for the most obvious pattern that applies to the majority. In this case, four numbers ($ 11, 77, 88, 55 $) have repeating digits, making $ 21 $ the clear outlier.

Q: Are there other ways to find patterns in number sequences?

Yes! You can look for divisibility rules, even/odd patterns, digit sums, or mathematical relationships. Always start with the most visible pattern first.

Q: What if I thought the outlier was 88 because it's the largest?

Size isn't always the pattern! While 88 is largest, three other numbers also don't follow size order. The digit repetition pattern is much stronger since it applies to 4 out of 5 numbers.

Q: How do I know which pattern is the 'right' one?

The correct pattern should apply to most numbers in the sequence (usually 4 out of 5, or 3 out of 4). The pattern with the clearest majority is typically the answer.

Q: Could 21 be special for other reasons?

While $ 21 = 3 \times 7 $ has other properties, the digit repetition pattern is most obvious here. In math problems, choose the simplest and most apparent pattern first.

Pattern Recognition with Digit Repetition

Which number is the outlier?

$21,11,77,88,55$

❤️ Continue Your Math Journey!

We have hundreds of course questions with personalized recommendations + Account 100% premium

Step-by-step written solution

Follow each step carefully to understand the complete solution

Understand the problem

Which number is the outlier?

$21,11,77,88,55$

Step-by-step solution

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 $21$ .

Final Answer

$21$

Key Points to Remember

Essential concepts to master this topic

Pattern Analysis: Look for common properties shared by most numbers
Technique: Compare digit structure: 11, 77, 88, 55 all repeat digits
Check: Verify outlier breaks the pattern: 21 has distinct digits 2≠1 ✓

Common Mistakes

Avoid these frequent errors

Comparing numbers by size or mathematical value
Don't compare 21, 11, 77, 88, 55 by which is largest or smallest = missing the real pattern! Size isn't the organizing principle here. Always look for structural patterns like digit repetition, divisibility, or number properties first.

Practice Quiz

Test your knowledge with interactive questions

What number do the blue squares below represent?

FAQ

Everything you need to know about this question

What if multiple numbers could be outliers?

Look for the most obvious pattern that applies to the majority. In this case, four numbers ( $11, 77, 88, 55$ ) have repeating digits, making $21$ the clear outlier.

Are there other ways to find patterns in number sequences?

Yes! You can look for divisibility rules, even/odd patterns, digit sums, or mathematical relationships. Always start with the most visible pattern first.

What if I thought the outlier was 88 because it's the largest?

Size isn't always the pattern! While 88 is largest, three other numbers also don't follow size order. The digit repetition pattern is much stronger since it applies to 4 out of 5 numbers.

How do I know which pattern is the 'right' one?

The correct pattern should apply to most numbers in the sequence (usually 4 out of 5, or 3 out of 4). The pattern with the clearest majority is typically the answer.

Could 21 be special for other reasons?

While $21 = 3 \times 7$ has other properties, the digit repetition pattern is most obvious here. In math problems, choose the simplest and most apparent pattern first.

🌟 Unlock Your Math Potential

Get unlimited access to all 18 Natural Numbers around 100 questions, detailed video solutions, and personalized progress tracking.

📹

Unlimited Video Solutions

Step-by-step explanations for every problem

📊

Progress Analytics

Track your mastery across all topics

🚫

Ad-Free Learning

Focus on math without distractions

No credit card required • Cancel anytime