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# What is the greatest prime factor of 4^17 - 2^28

Author Message
Intern
Joined: 17 Aug 2008
Posts: 12
What is the greatest prime factor of 4^17 - 2^28 [#permalink]

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14 Dec 2008, 12:09

What is the greatest prime factor of 4^17 - 2^28?
a) 2
b) 3
c) 5
d) 7
e) 11

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Intern
Joined: 17 Aug 2008
Posts: 12

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14 Dec 2008, 12:13
Never mind - I got it. factor out the 2^34.

Thanks anyway.
Intern
Joined: 05 Nov 2007
Posts: 35

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14 Dec 2008, 19:38
d)

2^28(2^6 - 1)
2^28 * 63

prime factors 2,3,7
Current Student
Joined: 11 Nov 2008
Posts: 107
Location: Indonesia
GMAT 1: 720 Q49 V40
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15 Dec 2008, 06:27
gmatatouille wrote:
d)

2^28(2^6 - 1)
2^28 * 63

prime factors 2,3,7

gmatatouille - thanks for the bolded formula! Now I know what's the quickest way to calculate it
Retired Moderator
Joined: 05 Jul 2006
Posts: 1734

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15 Dec 2008, 09:46
What is the greatest prime factor of 4^17 - 2^28?
a) 2
b) 3
c) 5
d) 7
e) 11

2^34 - 2^28 = 2^28(2^6 -1) = 64-1 = 63 = 3^2*7..........answer is 7

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
Re: GMAC question   [#permalink] 15 Dec 2008, 09:46
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# What is the greatest prime factor of 4^17 - 2^28

Moderator: chetan2u

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