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10! + 9! can be rewritten as:

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Math Expert
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Joined: 02 Sep 2009
Posts: 58449
10! + 9! can be rewritten as:  [#permalink]

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New post 25 Apr 2017, 02:16
00:00
A
B
C
D
E

Difficulty:

  35% (medium)

Question Stats:

69% (01:25) correct 31% (01:17) wrong based on 153 sessions

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Intern
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B
Joined: 13 Sep 2015
Posts: 24
Re: 10! + 9! can be rewritten as:  [#permalink]

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New post 25 Apr 2017, 02:27
1
10! + 9!
= (10*9!)+ 9!
=9! (10+1)
=9!(11)
=(9!*11*10)/10
=11!/10

Hence B

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Joined: 12 Sep 2015
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Re: 10! + 9! can be rewritten as:  [#permalink]

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New post 07 Mar 2018, 10:16
3
Top Contributor
Bunuel wrote:
10! + 9! can be rewritten as:

A. 9!(10)
B. 11!/10
C. (10!)^2*9
D. (9!)^2 + 10
E. 19!


First recognize that 10! = (10)(9!)
Here's why:
10! = (10)(9)(8)(7)(6)(5)(4)(3)(2)(1)
= (10)(9!)

So....
10! + 9! = (10)(9!) + 9!
= 9!(10 + 1) [I factored out the 9!]
= 9!(11)
Check the answer choice.....9!(11) isn't there! So, we'll need to check each answer choice to see which one can be expressed as (9!)(11)



A. 9!(10)
We can clearly see that this is not equal to (9!)(11)
ELIMINATE A


B. 11!/10
11!/10 = (11)(10)(9)(8)(7)(6)(5)(4)(3)(2)(1)/(10)
= (11)(9)(8)(7)(6)(5)(4)(3)(2)(1)
= (11)(9!) PERFECT!!
= B

Cheers,
Brent
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Re: 10! + 9! can be rewritten as:  [#permalink]

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New post 12 Mar 2018, 10:55
Bunuel wrote:
10! + 9! can be rewritten as:

A. 9!(10)
B. 11!/10
C. (10!)^2*9
D. (9!)^2 + 10
E. 19!


10! + 9! = 10 x 9! + 9! = 9!(10 + 1) = 9!(11) = 11!/10

Answer: B
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Re: 10! + 9! can be rewritten as:   [#permalink] 12 Mar 2018, 10:55
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