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Re: Is the 22nd term of an Arithmetic Progression odd? [#permalink]
This question is absolutely not GMAT-like. GMAC won't ask with such specific term "arithmetic progression".

Don't bother to answer the question

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Re: Is the 22nd term of an Arithmetic Progression odd? [#permalink]
IanStewart I missed the subtle case. Thanks for the explanation.

IanStewart wrote:
JayPatadiya wrote:
Is the 22nd term of an Arithmetic Progression odd?
1. The 24th tern is even
2. The first term is odd


The OA is given as A here, which is not correct. Using only Statement 1, the 22nd, 23rd and 24th terms of the sequence could be, say

22, 23, 24

in which case the 22nd term is even, but they could also be

21, 22.5, 24

in which case the 22nd term is odd.

As the question is written, the answer is C, since with both statements you can be certain the 22nd term is not odd. If you pretend the 22nd term is odd, you can see you'll reach a contradiction. If the list goes up by d from one term to the next:

- if the 22nd and 24th terms are integers in an equally spaced list, then 2d must be an integer, and every term in an even position in the sequence must be an integer
- so the 2nd term would then be an integer
- but we know the first term is an integer too, and if the 1st and 2nd terms are integers, every term is an integer, and the spacing d is an integer
- but then the 22nd and 24th terms would need to both be even or both be odd, since they are 2d apart. But they're not both even or both odd, so this entire situation is impossible.

So it's impossible, using both Statements, for the 22nd term to be odd, and the answer is C. The reasoning here is probably a lot more complicated than the question designer intended, if the question designer believed the answer was A. The answer is only A if the question tells you the terms in the sequence are integers.
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Re: Is the 22nd term of an Arithmetic Progression odd? [#permalink]
Expert Reply
JayPatadiya wrote:
Is the 22nd term of an Arithmetic Progression odd?

1. The 24th tern is even
2. The first term is odd


Solution:
Pre Analysis:
  • We are asked if the 22nd term of an AP is odd or not
  • This is a YES-NO question

Statement 1: The 24th tern is even
  • We know \(T_{22}=T_{24}-2d\) where d is the common difference
  • So, \(T_{22}=Even-Even=Even\)
  • Thus, statement 1 alone is sufficient and we can eliminate options B, C and E

Statement 2: The first term is odd
  • We know \(T_{22}=T_{1}+21d\)
  • So, \(T_{22}=Odd+21d\)
  • To predict the even-odd nature of \(T_{22}\), we will need the even-odd nature of \(d\)
  • Thus, statement 2 alone is not sufficient


Hence the right answer is Option A
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Re: Is the 22nd term of an Arithmetic Progression odd? [#permalink]
SaquibHGMATWhiz wrote:
JayPatadiya wrote:
Is the 22nd term of an Arithmetic Progression odd?

1. The 24th tern is even
2. The first term is odd


Solution:
Pre Analysis:
  • We are asked if the 22nd term of an AP is odd or not
  • This is a YES-NO question

Statement 1: The 24th tern is even
  • We know \(T_{22}=T_{24}-2d\) where d is the common difference
  • So, \(T_{22}=Even-Even=Even\)
  • Thus, statement 1 alone is sufficient and we can eliminate options B, C and E

Statement 2: The first term is odd
  • We know \(T_{22}=T_{1}+21d\)
  • So, \(T_{22}=Odd+21d\)
  • To predict the even-odd nature of \(T_{22}\), we will need the even-odd nature of \(d\)
  • Thus, statement 2 alone is not sufficient


Hence the right answer is Option A


How can we be sure that d is an integer ?
if d=1/2.

IMO: Answer should be C.
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Re: Is the 22nd term of an Arithmetic Progression odd? [#permalink]
Expert Reply
Nabneet wrote:
SaquibHGMATWhiz wrote:
JayPatadiya wrote:
Is the 22nd term of an Arithmetic Progression odd?

1. The 24th tern is even
2. The first term is odd


Solution:
Pre Analysis:
  • We are asked if the 22nd term of an AP is odd or not
  • This is a YES-NO question

Statement 1: The 24th tern is even
  • We know \(T_{22}=T_{24}-2d\) where d is the common difference
  • So, \(T_{22}=Even-Even=Even\)
  • Thus, statement 1 alone is sufficient and we can eliminate options B, C and E

Statement 2: The first term is odd
  • We know \(T_{22}=T_{1}+21d\)
  • So, \(T_{22}=Odd+21d\)
  • To predict the even-odd nature of \(T_{22}\), we will need the even-odd nature of \(d\)
  • Thus, statement 2 alone is not sufficient


Hence the right answer is Option A


How can we be sure that d is an integer ?
if d=1/2.

IMO: Answer should be C.


Hey Nabneet,

Thank you for pointing it out. I completely missed out on this constraint.
Apart from that, this question is a little complicatedly designed and is well explained by IanStewart.
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Re: Is the 22nd term of an Arithmetic Progression odd? [#permalink]
How do we know the common difference is an integer?
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Re: Is the 22nd term of an Arithmetic Progression odd? [#permalink]
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