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FDPs -- Help Evaluating Factorials

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FDPs -- Help Evaluating Factorials

by laura003 » Fri Apr 23, 2010 7:16 am
Hi All,

I'm a new poster -- but long-time reader -- of the BTG Forum looking for help on this question from a recent MGMAT CAT exam. I know the answer is D once you convert them all to a common denominator of 5(9!), but I'm struggling to understand why.

Could someone please offer up an explanation for why D is the correct choice? Specifically, the MGMAT CAT explanation says you should multiply answer choice D by 8 and 9 to get the common base -- and I don't understand why that 8 gets thrown in.

Thanks for your help!
Laura


Which of the following has the greatest value?
(A) 3/8!
(B) 6(4!)
5(9!)
(C) 4(3!)
9!
(D) 3
5(7!)
(E) 14
5(8!)
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by akhpad » Fri Apr 23, 2010 7:20 am
Answer choices are not in proper way.

Can you please arrange it?
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by laura003 » Fri Apr 23, 2010 7:23 am
Sorry about that. In the problem, MGMAT doesn't use the fraction bar on choices B, C, D, and E, it just has one number on top of the other w/o the bar in the middle, so I was trying to mimic that. For the purposes of clarity and legibility though, I'll add in the fraction bar on all the answer choices.

Which of the following has the greatest value?
(A) 3/8!
(B) 6(4!)/5(9!)
(C) 4(3!)/9!
(D) 3/5(7!)
(E) 14/5(8!)

Thanks!
Laura
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by akhpad » Fri Apr 23, 2010 7:27 am
(A) 3/8! = 135 / ( 5 * 9! )
(B) 6(4!)/5(9!) = 144 / ( 5 * 9! )
(C) 4(3!)/9! = 120 / ( 5 * 9! )
(D) 3/5(7!) = 216 / ( 5 * 9! )
(E) 14/5(8!) = 126 / ( 5 * 9! )


Hence D
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by laura003 » Fri Apr 23, 2010 7:32 am
Thanks for your quick reply. Why did you multiply by 8 on answer choice (D) to get 216/5(9!)?

I understand that you needed to multiply by 9 to get the 5(9!) denominator, but I'm not sure why the 8 was also necessary.

Thank you!
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by akhpad » Fri Apr 23, 2010 7:36 am
For comparison, it is better to compare in integer rather than fraction.

I made 5 * 9! at denominator.

All nominator become integer.
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by Stuart@KaplanGMAT » Fri Apr 23, 2010 4:15 pm
laura003 wrote:Sorry about that. In the problem, MGMAT doesn't use the fraction bar on choices B, C, D, and E, it just has one number on top of the other w/o the bar in the middle, so I was trying to mimic that. For the purposes of clarity and legibility though, I'll add in the fraction bar on all the answer choices.

Which of the following has the greatest value?
(A) 3/8!
(B) 6(4!)/5(9!)
(C) 4(3!)/9!
(D) 3/5(7!)
(E) 14/5(8!)

Thanks!
Laura
Hi,

for (D), we're changing 7! to 9!.

Well, 9! = 9*8*7!

So, to get a denominator of 9!, we need to multiply the top and bottom of the fraction by 9*8:

3/5(7!) * (9*8)/(9*8)

= 3*9*8/5*(9*8*7!)

= 216 /5*(9!)
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by laura003 » Sat Apr 24, 2010 5:28 am
Thank you both!
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by govind_raj_76 » Sat Apr 24, 2010 5:50 am
Akhilesh

How did u arrive at a common number ? Did u go by taking the max of dr 5(9!) in option B , made that a reference to change other options A,C,D and E ?

(A) 3/8! = 135 / ( 5 * 9! )
(B) 6(4!)/5(9!) = 144 / ( 5 * 9! )
(C) 4(3!)/9! = 120 / ( 5 * 9! )
(D) 3/5(7!) = 216 / ( 5 * 9! )
(E) 14/5(8!) = 126 / ( 5 * 9! )

Thanks,
Govind
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by akhpad » Sat Apr 24, 2010 5:59 am
Yes, we can say max of Denominators in this case.

But in general, if we have to compare fractional numbers, take the LCM of denominators and make the denominator common (LCM) so that we can compare Nominator only.

Usually, we should not play with the factional numbers as this is a time consuming.
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