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For integers x, y, and z, if ((2^x)^y)^z = 131072 which of

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Intern
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For integers x, y, and z, if ((2^x)^y)^z = 131072 which of [#permalink]

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03 Oct 2013, 23:03
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For integers x, y, and z, if $$((2^x)^y)^z = 131072$$ which of the following must be true ?

A. The product xyz is even
B. The product xyz is odd
C. The product xy is even
D. The product yz is prime
E. The product yz is positive

I can understand that the last digit will be odd (i.e. z = odd) but how to determine the rest, x & y ?
[Reveal] Spoiler: OA

Last edited by Bunuel on 03 Oct 2013, 23:57, edited 1 time in total.
Renamed the topic and edited the question.
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Re: Veritas Prep PS 17 - For integers x, y, and z [#permalink]

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03 Oct 2013, 23:32
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It is a PS problem.

The problems comes down to 2^(xyz) = 131072
2^10 = 1024 ( Good to remember this ) So if you count the powers 11 = 2***, 12 = 4***. 13 = 8***, so on

We find 2^17 = 131072, which imples xyz = 17

Hence the best possible solution is B. xyz is odd .
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Re: Veritas Prep PS 17 - For integers x, y, and z [#permalink]

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03 Oct 2013, 23:33
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joylive wrote:
For integers x, y, and z, if $$((2^x)^y)^z = 131072$$ which of the following must be true ?
• The product xyz is even
• The product xyz is odd
• The product xy is even
• The product yz is prime
• The product yz is positive

I can understand that the last digit will be odd (i.e. z = odd) but how to determine the rest, x & y ?

The number 2 has the cyclicity of 4 means:
2^1 =last digit = 2
2^2 = last digit =4
2^3 =last digit = 8
2^4 = last digit = 6

now after this again this will repeat.
so every 4k + 1 term will have the unit digit = 2
now in our question last digit is 2 so x*y*z will be of form 4k + 1 = ODD

hence B
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Re: Veritas Prep PS 17 - For integers x, y, and z [#permalink]

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03 Oct 2013, 23:37
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joylive wrote:
For integers x, y, and z, if $$((2^x)^y)^z = 131072$$ which of the following must be true ?
• The product xyz is even
• The product xyz is odd
• The product xy is even
• The product yz is prime
• The product yz is positive

I can understand that the last digit will be odd (i.e. z = odd) but how to determine the rest, x & y ?

$$((2^x)^y)^z = 2^{xyz} =$$ (the powers get multiplied)

Now look at the last digit of 131072 and think of the cyclicity of 2.
$$2^1 = 2$$
$$2^2 = 4$$
$$2^3 = 8$$
$$2^4 = 16$$
$$2^5 = 32$$
$$2^6 = 64$$

Since 131072 ends with a 2, the power xyz must be of the form (4n + 1) i.e. it must be 1 more than a multiple of 4 (since the cyclicity of 2 is 4). Since xyz is one more than a multiple of 4, it must be odd.

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Get started with Veritas Prep GMAT On Demand for $199 Veritas Prep Reviews Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 7988 Location: Pune, India Re: Veritas Prep PS 17 - For integers x, y, and z [#permalink] Show Tags 03 Oct 2013, 23:47 1 This post received KUDOS Expert's post joylive wrote: blueseas wrote: joylive wrote: For integers x, y, and z, if $$((2^x)^y)^z = 131072$$ which of the following must be true ? • The product xyz is even • The product xyz is odd • The product xy is even • The product yz is prime • The product yz is positive I can understand that the last digit will be odd (i.e. z = odd) but how to determine the rest, x & y ? The number 2 has the cyclicity of 4 means: 2^1 =last digit = 2 2^2 = last digit =4 2^3 =last digit = 8 2^4 = last digit = 6 now after this again this will repeat. so every 4k + 1 term will have the unit digit = 2 now in our question last digit is 2 so x*y*z will be of form 4k + 1 = ODD hence B Thanks for replying - still not getting the question. I'm aware of Last Digit cycle of 2 , that is how z is odd, but the product is 2 ^ (xyz), we know z = odd, but how do we figure out x and y to answer the question, finding out the the powers of 2 till 131072 is definitely not an option in GMAT, i suppose Ok, why don't you try this: Say x = 2, y = 3, z = 5 (z is odd) $$((2^2)^3)^5 = 2^{30}$$ What digit do you think this will end with? I hope you will agree it will end with 4. Notice that the last digit is decided by the entire power of 2. Only z being odd is not enough. If either one of x or y is even, the entire power will become even and then it will end with 4 or 6 _________________ Karishma Veritas Prep | GMAT Instructor My Blog Get started with Veritas Prep GMAT On Demand for$199

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Re: Veritas Prep PS 17 - For integers x, y, and z [#permalink]

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03 Oct 2013, 23:51
joylive wrote:
For integers x, y, and z, if $$((2^x)^y)^z = 131072$$ which of the following must be true ?
• The product xyz is even
• The product xyz is odd
• The product xy is even
• The product yz is prime
• The product yz is positive

I can understand that the last digit will be odd (i.e. z = odd) but how to determine the rest, x & y ?

joy ,
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Re: Veritas Prep PS 17 - For integers x, y, and z [#permalink]

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03 Oct 2013, 23:57
VeritasPrepKarishma wrote:
The number 2 has the cyclicity of 4 means:
2^1 =last digit = 2
2^2 = last digit =4
2^3 =last digit = 8
2^4 = last digit = 6

Ok, why don't you try this:
Say x = 2, y = 3, z = 5 (z is odd)

$$((2^2)^3)^5 = 2^{30}$$
What digit do you think this will end with? I hope you will agree it will end with 4.

Notice that the last digit is decided by the entire power of 2. Only z being odd is not enough. If either one of x or y is even, the entire power will become even and then it will end with 4 or 6

Thanks @Karishma, my confusion was the product xyz, thought it is asking x*y*z = odd, thanks to you all, it's resolved now.
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Re: For integers x, y, and z, if ((2^x)^y)^z = 131072 which of [#permalink]

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31 Jan 2016, 18:42
it is somehow way too time consuming to be an actual gmat type question...
it all comes down to identify how many powers of 2 we have..
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Re: For integers x, y, and z, if ((2^x)^y)^z = 131072 which of [#permalink]

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31 Jan 2016, 20:20
mvictor wrote:
it is somehow way too time consuming to be an actual gmat type question...
it all comes down to identify how many powers of 2 we have..

No. If the question wanted you to find the powers of 2, then the number on the right hand side would have been something like 512 or 1024... May be 2048 or 4096 - something you are reasonably expected to know or figure out quickly. The question expects you to understand the concept of cyclicity and exponents multiplication. Then it takes no more than a few secs to arrive at the answer.
Look at my solution here: for-integers-x-y-and-z-if-2-x-y-z-131072-which-of-160992.html#p1274052
If you are not sure about cyclicity, check out this post: http://www.veritasprep.com/blog/2015/11 ... -the-gmat/
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Get started with Veritas Prep GMAT On Demand for $199 Veritas Prep Reviews Board of Directors Joined: 17 Jul 2014 Posts: 2752 Location: United States (IL) Concentration: Finance, Economics GMAT 1: 650 Q49 V30 GPA: 3.92 WE: General Management (Transportation) Re: For integers x, y, and z, if ((2^x)^y)^z = 131072 which of [#permalink] Show Tags 01 Feb 2016, 08:06 VeritasPrepKarishma wrote: mvictor wrote: it is somehow way too time consuming to be an actual gmat type question... it all comes down to identify how many powers of 2 we have.. No. If the question wanted you to find the powers of 2, then the number on the right hand side would have been something like 512 or 1024... May be 2048 or 4096 - something you are reasonably expected to know or figure out quickly. The question expects you to understand the concept of cyclicity and exponents multiplication. Then it takes no more than a few secs to arrive at the answer. Look at my solution here: for-integers-x-y-and-z-if-2-x-y-z-131072-which-of-160992.html#p1274052 If you are not sure about cyclicity, check out this post: http://www.veritasprep.com/blog/2015/11 ... -the-gmat/ You are right...nevertheless, with such a big number, I started to think that xyz is a fraction. Dumb me, did not pay attention to the answer choices.. I started to find the prime factorization...and it took way too much time.. Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 7988 Location: Pune, India Re: For integers x, y, and z, if ((2^x)^y)^z = 131072 which of [#permalink] Show Tags 01 Feb 2016, 08:35 1 This post received KUDOS Expert's post mvictor wrote: You are right...nevertheless, with such a big number, I started to think that xyz is a fraction. Dumb me, did not pay attention to the answer choices.. I started to find the prime factorization...and it took way too much time.. For future reference, remember that in GMAT Quant you are not expected to mess around with such an unwieldy number. If they want you to prime factorise it, they will not go beyond a 3 digit number. If it is larger, then the number would have factors that you can instantly see (a number such as 10000 etc). In case of such large numbers, focus on last digit/ odd-even / simplification (such as 9999 = 10000 - 1) etc. _________________ Karishma Veritas Prep | GMAT Instructor My Blog Get started with Veritas Prep GMAT On Demand for$199

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Re: For integers x, y, and z, if ((2^x)^y)^z = 131072 which of [#permalink]

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25 Sep 2017, 05:06
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Re: For integers x, y, and z, if ((2^x)^y)^z = 131072 which of   [#permalink] 25 Sep 2017, 05:06
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