Number Series, Coding and DecodingMCQMTP June 22Question 2190 of 217
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Find wrong number of the series 1,5,9,7,11,15,12,17\displaystyle 1, 5, 9, 7, 11, 15, 12, 17

Options

A11\displaystyle 11
B12\displaystyle 12
C17\displaystyle 17
D15\displaystyle 15
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Correct Answer

✅ Option b — 12\displaystyle 12

All Options:

  • A11\displaystyle 11
  • B12\displaystyle 12
  • C17\displaystyle 17
  • D15\displaystyle 15

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Detailed Solution & Explanation

Let's analyze the structure of the series: 1,5,9,7,11,15,12,17\displaystyle 1, 5, 9, 7, 11, 15, 12, 17.
The series consists of groups of three numbers where each group starts at an odd number and has a step difference of +4\displaystyle +4:
- **Group 1**: 1,5,9\displaystyle 1, 5, 9 (starts at 1\displaystyle 1, common difference +4\displaystyle +4)
- **Group 2**: 7,11,15\displaystyle 7, 11, 15 (starts at 7\displaystyle 7, common difference +4\displaystyle +4)
- **Group 3**: 13,17,21\displaystyle 13, 17, 21 (starts at 13\displaystyle 13, common difference +4\displaystyle +4)

If we write the full merged series, we get: 1,5,9,7,11,15,13,17,21\displaystyle 1, 5, 9, 7, 11, 15, \mathbf{13}, 17, 21.
Comparing this to the given series, the term 12\displaystyle 12 is incorrect and should be 13\displaystyle 13. Therefore, 12\displaystyle 12 is the wrong term.

Hence, **Option B** is the correct answer.

About This Chapter: Number Series, Coding and Decoding

Paper

Paper 3: Quantitative Aptitude

Weightage

5 Marks

Key Topics

Number series, Coding & Decoding, Odd man out

This chapter covers Number series, Coding & Decoding, Odd man out and is part of Paper 3: Quantitative Aptitude in the CA Foundation exam.

View Official ICAI Syllabus

Exam Strategy Tip

This topic carries 5 Marks weightage. Focus on understanding core concepts rather than memorizing.

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