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10! + 9! can be rewritten as:

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by vinni.k » Wed Mar 07, 2018 8:11 am
10! + 9! can be rewritten as:

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

OA is B

My approach
10! + 9!
10*9! + 9!
Taking out 9! common
9!(10 + 1)
9!(11)
Now from here i don't know how to proceed. So, i am bit stuck here. However, when i was reviewing, i played around with B
11!/10
(11 * 10 * 9!) / 10
(11)9!
Same outcome as what i got when i solved earlier, but how to get B on my first approach.

Thanks
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by Brent@GMATPrepNow » Wed Mar 07, 2018 9:05 am
vinni.k wrote:10! + 9! can be rewritten as:

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

OA is B

My approach
10! + 9!
10*9! + 9!
Taking out 9! common
9!(10 + 1)
9!(11)
Now from here i don't know how to proceed. So, i am bit stuck here. However, when i was reviewing, i played around with B
11!/10
(11 * 10 * 9!) / 10
(11)9!
Same outcome as what i got when i solved earlier, but how to get B on my first approach.

Thanks
You're almost there!

You have (11)9!
Check the answer choice.....(11)9! isn't there! So, we'll need to check each answer choice.

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

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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by vinni.k » Wed Mar 07, 2018 9:32 am
Brent@GMATPrepNow wrote:
You're almost there!

You have (11)9!
Check the answer choice.....(11)9! isn't there! So, we'll need to check each answer choice.
Thanks Brent,
So, the right approach for this question is to check each of the answer choices.
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by Brent@GMATPrepNow » Wed Mar 07, 2018 10:21 am
vinni.k wrote:
Brent@GMATPrepNow wrote:
You're almost there!

You have (11)9!
Check the answer choice.....(11)9! isn't there! So, we'll need to check each answer choice.
Thanks Brent,
So, the right approach for this question is to check each of the answer choices.
Once you've rewritten 10! + 9! as (11)9! and discovered that your answer is not among the answer choices, you can EITHER try to rewrite (11)9! so that it matches one of the answer choices, OR you can try to rewrite each answer choice so that it matches (11)9!.
The second option is most likely the faster approach.

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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by Jeff@TargetTestPrep » Mon Mar 12, 2018 10:33 am
vinni.k 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

Jeffrey Miller
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