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If n is the greatest positive integer for which 2n is a fact

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If n is the greatest positive integer for which 2n is a fact [#permalink] New post 25 Dec 2012, 09:05
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If n is the greatest positive integer for which 2^n is a factor of 10!, then n =?

A. 2
B. 4
C. 6
D. 8
E. 10

Is any one can provide a solution for this question?It's from GWD. Thanks!
[Reveal] Spoiler: OA
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Re: If n is the greatest positive integer for which 2n is a fact [#permalink] New post 25 Dec 2012, 09:18
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In the questions where you are supposed to find out the maximum power of a prime factor which is a factor of n!, keep one only on thing in mind:
Let the prime number you are looking for be x, then:
\(n/x + n/x^2 + n/x^3.........n/x^k\) where \(x^k\) <=n.
If you apply this rule here,
\(10/2 +10/2^2 +10/2^3\) or 5+2+1=8.
Hence maximum power of 2 in \(10!\) will be 8.
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Re: If n is the greatest positive integer for which 2n is a fact [#permalink] New post 26 Dec 2012, 02:49
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curtis0063 wrote:
If n is the greatest positive integer for which 2^n is a factor of 10!, then n =?

A. 2
B. 4
C. 6
D. 8
E. 10

Is any one can provide a solution for this question?It's from GWD. Thanks!


Check here: everything-about-factorials-on-the-gmat-85592.html
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Re: If n is the greatest positive integer for which 2n is a fact [#permalink] New post 09 Jan 2014, 09:44
We can count the total number of powers of 2 in 10!
There are 8 of them.So answer D.

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Re: If n is the greatest positive integer for which 2n is a fact [#permalink] New post 09 Jan 2014, 12:22
If n is the greatest positive integer for which 2^n is a factor of 10!, then n =?

A. 2
B. 4
C. 6
D. 8
E. 10


Solution:
On a closer look this Qs is asking how many 2s are there in multiplication of 10!
Now, as we know 10! can be written as -- 10.9.8.7.6.5.4.3.2.1

As this is a small multiplication ( To review , not to do), we can check how many 2s we have in this..,
So, 10-1 (no.of 2s), 8-3(no.of 2s), 6-1(no.of 2s),4-2(no.of 2s),2-1(no.of 2s)

Adding all, we'll have 1+3+1+2+1=8

Hence, ans. D.

Hope this helps
Dj. :wink:
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Re: If n is the greatest positive integer for which 2n is a fact [#permalink] New post 14 May 2015, 14:05
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Re: If n is the greatest positive integer for which 2n is a fact [#permalink] New post 14 May 2015, 22:06
Expert's post
Hi All,

Many Test Takers get these types of questions wrong because they move too quickly through the work and don't do enough work on their pads. It's a relatively straight-forward prompt though...

We're essentially asked to find all the "2"s inside 10!

The 'key' to this question is to realize that some values have MORE THAN ONE 2 in them....

10! = (10)(9)(8)(7)(6)(5)(4)(3)(2)(1)

10 = 5x2 --> one 2
8 = 2x2x2 --> three 2s
6 = 3x2 --> one 2
4 = 2x2 --> two 2s
2 = 1x2 --> one 2

1+3+1+2+1 = eight 2s

Final Answer:
[Reveal] Spoiler:
D


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Re: If n is the greatest positive integer for which 2n is a fact   [#permalink] 14 May 2015, 22:06
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