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Find the number of trailing zeros in the expansion of

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Joined: 05 May 2019
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Re: Find the number of trailing zeros in the expansion of  [#permalink]

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New post 24 Aug 2019, 08:58
Bunuel wrote:
Sachin9 wrote:
do we get such questions on gmat?


Trailing zeros concept is tested on the GMAT (check here: http://gmatclub.com/forum/everything-ab ... 85592.html), though this particular question is a bit out of scope.


Hey Bunuel. Old post but, nevertheless.
What really makes a question out of scope? The concept tested here definitely is included on the GMAT. Difficulty? Time to solve a question? complexity? number of concepts related to the question?

Just for the sake of everybody's clarity.
I've read a lot of great things about you.
I'm a newcomer here. So, kindly enlighten me!
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Re: Find the number of trailing zeros in the expansion of  [#permalink]

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New post 26 Aug 2019, 01:37
sharathnair14 wrote:
Bunuel wrote:
Sachin9 wrote:
do we get such questions on gmat?


Trailing zeros concept is tested on the GMAT (check here: http://gmatclub.com/forum/everything-ab ... 85592.html), though this particular question is a bit out of scope.


Hey Bunuel. Old post but, nevertheless.
What really makes a question out of scope? The concept tested here definitely is included on the GMAT. Difficulty? Time to solve a question? complexity? number of concepts related to the question?

Just for the sake of everybody's clarity.
I've read a lot of great things about you.
I'm a newcomer here. So, kindly enlighten me!


I think a little bit of everything you've mentioned. If not the shortcut mentioned in my post, the question would require more time than 2 minutes on average. Also, you can see in our stats that 43% of users out of 1442, as of today, solve the question incorrectly. Which is quite a big percentage of wrong answers. Those who solved correctly spent 02:46 on average.

Plus the wording is not the one GMAC would use. They wouldn't use "trailing zeros" (you are not supposed to know what a trailing zero is), instead the'd say something like "the number of zeros after rightmost non-zero digit of ..."
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Re: Find the number of trailing zeros in the expansion of  [#permalink]

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New post 26 Aug 2019, 08:31
feruz77 wrote:
Find the number of trailing zeros in the expansion of (20!*21!*22! ……… *33!)^3!.

A. 468
B. 469
C. 470
D. 467
E. 471

Can someone help me how to solve this question? I think, there must be more than one solution method.

Do questions of such a level of difficulty appear on the actual GMAT?


Asked: Find the number of trailing zeros in the expansion of (20!*21!*22! ……… *33!)^3!.

(20!*21!*22! ……… *33!)^3!.

Highest Power of 5 in 20! = 4
Highest Power of 5 in 21! = 4
Highest Power of 5 in 22! = 4
Highest Power of 5 in 23! = 4
Highest Power of 5 in 24! = 4
Highest Power of 5 in 25! = 5 + 1 = 6
Highest Power of 5 in 26! = 5 + 1 = 6
Highest Power of 5 in 27! = 5 + 1 = 6
Highest Power of 5 in 28! = 5 + 1 = 6
Highest Power of 5 in 29! = 5 + 1 = 6
Highest Power of 5 in 30! = 6 + 1 = 7
Highest Power of 5 in 31! = 6 + 1 = 7
Highest Power of 5 in 32! = 6 + 1 = 7
Highest Power of 5 in 33! = 6 + 1 = 7

Number of trailing zeros in (20!*21!*22! ……… *33!)^3!. = (4*5 + 6*5 + 4*7)*6 = (20 + 30 + 28)*6 = 78*6 = 468

IMO A
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Re: Find the number of trailing zeros in the expansion of   [#permalink] 26 Aug 2019, 08:31

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