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# Which of the following is equal to (2^k) ? A. 2(10^k 1) B.

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Senior Manager
Joined: 10 Mar 2008
Posts: 342
Which of the following is equal to (2^k) ? A. 2(10^k 1) B. [#permalink]

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01 Oct 2008, 11:11
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Which of the following is equal to (2^k) [5^(k − 1)]?

A. 2(10^k − 1)
B. 5(10^k − 1)
C. 10^k
D. 2(10^k )
E. 10^2k − 1

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Manager
Joined: 22 Sep 2008
Posts: 119

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01 Oct 2008, 11:34

given eq = 2^5*5^k -2^k = 10^k -2^k
SVP
Joined: 07 Nov 2007
Posts: 1760
Location: New York

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01 Oct 2008, 11:35
vksunder wrote:
Which of the following is equal to (2^k)(5^k − 1)?

A. 2(10^k − 1)
B. 5(10^k − 1)
C. 10^k
D. 2(10^k )
E. 10^2k − 1

2^k*5^{k − 1}= $$2*2^{k-1}*5^{k-1} =2*10^{k-1}$$

A
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01 Oct 2008, 11:41
can you please re-post the question with the () clearly posted..i cant really follow the question as posted..
Director
Joined: 27 Jun 2008
Posts: 526
WE 1: Investment Banking - 6yrs

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01 Oct 2008, 11:44
Me neither - I could not understand the question?

I tried working with plugin numbers
Let k=2

2^2(5^2 – 1) = 4( 24) = 96

A 2*99 = gone
B 5*99 = gone
C 100 = gone
D 2*100 = gone
E 10^2*2-1 = gone

Confused!!
SVP
Joined: 07 Nov 2007
Posts: 1760
Location: New York

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01 Oct 2008, 12:04
(2^k)(5^k − 1)?
should be $$2^k 5^{k-1}$$
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Senior Manager
Joined: 04 Aug 2008
Posts: 369

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01 Oct 2008, 12:15
plug in should work too

replace k with 2

u should get 2^2*5^(2-1) = 4*5=20

choice A also produces 20 when k is replaced w 2

2*(10)=20
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VP
Joined: 05 Jul 2008
Posts: 1369

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01 Oct 2008, 12:49
x2suresh wrote:
(2^k)(5^k − 1)?
should be $$2^k 5^{k-1}$$

Yep there is a lot of different between $$2^k 5^{k-1}$$ & $$2^k {5^k -1}$$
Senior Manager
Joined: 10 Mar 2008
Posts: 342

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01 Oct 2008, 13:32
Suresh - thanks for correcting the error and also for the explanation.

Sorry for the confusion guys!
Senior Manager
Joined: 18 Jun 2007
Posts: 280

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01 Oct 2008, 14:20
lol me too the did make the mistake but when I rechecked it by plugging numbers it was clear.

question can be written as

2*{2^(k-1)*5^(k-1)}
2*{10^(k-1)} ........ans A
VP
Joined: 18 May 2008
Posts: 1203

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01 Oct 2008, 18:09
Guys, shldnt the answer choice A be 2(10^(k-1))?
There is a difference b/w 2(10^(k-1)) and 2(10^k − 1)
vksunder wrote:
Which of the following is equal to (2^k) [5^(k − 1)]?

A. 2(10^k − 1)
B. 5(10^k − 1)
C. 10^k
D. 2(10^k )
E. 10^2k − 1
VP
Joined: 17 Jun 2008
Posts: 1322

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01 Oct 2008, 22:42
vksunder wrote:
Which of the following is equal to (2^k) [5^(k − 1)]?

A. 2(10^k − 1)
B. 5(10^k − 1)
C. 10^k
D. 2(10^k )
E. 10^2k − 1

IMO A
10^k /5 => 10 * 10^k /5 => 2* 10^(k-1) => A is answer

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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.

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Re: Exponents   [#permalink] 01 Oct 2008, 22:42
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# Which of the following is equal to (2^k) ? A. 2(10^k 1) B.

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