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For any integer n, denotes the product of all of the

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Manager
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For any integer n, denotes the product of all of the [#permalink] New post 25 Jan 2008, 22:39
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A
B
C
D
E

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For any integer n, [n] denotes the product of all of the positive even numbers' from 2 to n. What is the largest prime factor of [23] + [21]?

A) 23
B) 19
C) 11
D) 7
E) 5

Do you have shortcut to solve this problem?

OA is A.
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Re: PS - Product of all the positive even numbers [#permalink] New post 25 Jan 2008, 23:51
Expert's post
A

[23] + [21] = [22] + [20] = [20]*(22+1) = 2*10!*23
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Re: PS - Product of all the positive even numbers [#permalink] New post 26 Jan 2008, 08:44
It must be A (23), because the balance possible prime numbers are in both series.

A too!!!
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Re: PS - Product of all the positive even numbers [#permalink] New post 26 Jan 2008, 15:54
i dont understand either explanation...

i could do the grunt math out but theres gotta bea faster way that im not seeing
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Re: PS - Product of all the positive even numbers [#permalink] New post 26 Jan 2008, 17:54
GMAT4Lyfe wrote:
i dont understand either explanation...

i could do the grunt math out but theres gotta bea faster way that im not seeing


[23]=2*4*6*8*10*12*14*16*18*20*22
[21]=2*4*6*8*10*12*14*16*18*20

[23]+[21]=2*4*6*8*10*12*14*16*18*20*(22+1)
[23]+[21]=2*4*6*8*10*12*14*16*18*20*23
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Re: PS - Product of all the positive even numbers   [#permalink] 26 Jan 2008, 17:54
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