Number Series, Coding and DecodingMCQPYQ Dec. 22Question 2169 of 217
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Find the odd man out: $34, 105, 424, 2123, 12756$.

Select Your Answer (Keyboard: A, B, C, D)
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Correct Answer

✅ Option b — $2123$

All Options:

  • A$12756$
  • B$2123$
  • C$424$
  • D$34$

Detailed Solution & Explanation

Let's establish the mathematical pattern linking consecutive terms starting with T1=34\displaystyle T_1 = 34:
- T2=T1×3+3=34×3+3=102+3=105\displaystyle T_2 = T_1 ×\displaystyle \times 3 + 3 = 34 ×\displaystyle \times 3 + 3 = 102 + 3 = 105
- T3=T2×4+4=105×4+4=420+4=424\displaystyle T_3 = T_2 ×\displaystyle \times 4 + 4 = 105 ×\displaystyle \times 4 + 4 = 420 + 4 = 424
- T4=T3×5+5=424×5+5=2120+5=2125\displaystyle T_4 = T_3 ×\displaystyle \times 5 + 5 = 424 ×\displaystyle \times 5 + 5 = 2120 + 5 = 2125
- T5=T4×6+6=2125×6+6=12750+6=12756\displaystyle T_5 = T_4 ×\displaystyle \times 6 + 6 = 2125 ×\displaystyle \times 6 + 6 = 12750 + 6 = 12756

This pattern (Tn=Tn−1×(n+1)+(n+1)\displaystyle T_n = T_{n-1} ×\displaystyle \times (n+1) + (n+1)) is perfectly consistent for all terms except $2123$, which should be $2125$. Therefore, $2123$ is the odd man out.

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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