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Manager  Joined: 20 Dec 2013
Posts: 222
Location: India
If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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31 00:00

Difficulty:   95% (hard)

Question Stats: 32% (02:17) correct 68% (02:00) wrong based on 359 sessions

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If n is an odd natural number and n! ends with 32 zeros, then how many values of n are possible?

A. 2
B. 3
C. 1
D. 4
E. none

Spoiler: :: My doubt & OA
I think the answer should be B(135,137,139).But the OA is A.Can anyone explain?

Originally posted by AKG1593 on 08 Jan 2014, 09:31.
Last edited by Bunuel on 09 Jan 2014, 01:57, edited 1 time in total.
Renamed the topic and edited the question.
Math Expert V
Joined: 02 Sep 2009
Posts: 60467
Re: If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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Intern  B
Joined: 17 Oct 2013
Posts: 40
Schools: HEC Dec"18
GMAT Date: 02-04-2014

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7
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AKG1593 wrote:
I saw this question online:
If n is an odd natural number and n! ends with 32 zeros, then how many values of n are possible? Note : n! means product of first n natural numbers.
a. 2
b. 3
c. 1
d. 4
e. none

Spoiler: :: My doubt & OA
I think the answer should be B(135,137,139).But the OA is A.Can anyone explain?

in n! the zeros are contributed by 2s and 5s, there will be adequate no of 2s for 5s. if we count no of 5s, we can find no of zeros.
if n! is written in the product of prime factors there should be 5^32 to get you 32 zeros
for 100!, it will be 5^24, hence counting each 5 from there
105(1),110(1),115(1),120(1),125(3),130(1) contribute 8 5s
since n is 131! and 133! will have 32 zeros
##### General Discussion
Manager  Joined: 20 Dec 2013
Posts: 222
Location: India

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Thanks for this reply.It seems I was missing 1 of the fives in 125!

Posted from my mobile device
Manager  B
Joined: 10 Mar 2014
Posts: 177
Re: If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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AKG1593 wrote:
If n is an odd natural number and n! ends with 32 zeros, then how many values of n are possible?

A. 2
B. 3
C. 1
D. 4
E. none

Spoiler: :: My doubt & OA
I think the answer should be B(135,137,139).But the OA is A.Can anyone explain?

Bunuel

i am not agree with option A as it is given as correct answer.

i am getting 135!, 137! and 139! for 32 trailing zeros. so i think it should be 3.

Thanks
Math Expert V
Joined: 02 Sep 2009
Posts: 60467
If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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2
3
PathFinder007 wrote:
AKG1593 wrote:
If n is an odd natural number and n! ends with 32 zeros, then how many values of n are possible?

A. 2
B. 3
C. 1
D. 4
E. none

Spoiler: :: My doubt & OA
I think the answer should be B(135,137,139).But the OA is A.Can anyone explain?

Bunuel

i am not agree with option A as it is given as correct answer.

i am getting 135!, 137! and 139! for 32 trailing zeros. so i think it should be 3.

Thanks

131! has 32 trailing zeros: 131/5 + 131/25 + 131/125 = 26 + 5 + 1 = 32.
133! has 32 trailing zeros: 133/5 + 133/25 + 133/125 = 26 + 5 + 1 = 32.

But 135! has 33 trailing zeros: 135/5 + 135/25 + 135/125 = 27 + 5 + 1 = 33.
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Math Expert V
Joined: 02 Sep 2009
Posts: 60467
Re: If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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1
1
Bunuel wrote:
PathFinder007 wrote:
AKG1593 wrote:
If n is an odd natural number and n! ends with 32 zeros, then how many values of n are possible?

A. 2
B. 3
C. 1
D. 4
E. none

Spoiler: :: My doubt & OA
I think the answer should be B(135,137,139).But the OA is A.Can anyone explain?

Bunuel

i am not agree with option A as it is given as correct answer.

i am getting 135!, 137! and 139! for 32 trailing zeros. so i think it should be 3.

Thanks

131! has 32 trailing zeros: 131/5 + 131/25 + 131/125 = 26 + 5 + 1 = 32.
133! has 32 trailing zeros: 133/5 + 133/25 + 133/125 = 26 + 5 + 1 = 32.

But 135! has 33 trailing zeros: 135/5 + 133/25 + 133/125 = 27 + 5 + 1 = 33.

For similiar questions check Trailing Zeros Questions and Power of a number in a factorial questions in our Special Questions Directory.

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Intern  Joined: 06 Apr 2015
Posts: 30
Re: If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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AKG1593 wrote:
If n is an odd natural number and n! ends with 32 zeros, then how many values of n are possible?

A. 2
B. 3
C. 1
D. 4
E. none

Spoiler: :: My doubt & OA
I think the answer should be B(135,137,139).But the OA is A.Can anyone explain?

I got the right answer but it took me close to 4 minutes to get to it.

by trial and error i plugged in first 100 and realized i was too low, then went to 125 and realized i was still too low, Finally went to 131 and hit jackpot.
I got the answer by using the below formula
(n/5) +(n/5^2) + (n/5^3) = 32

the final numbers are 131 and 133 (since they are odd Numbers only)

Is there a faster solution to this?
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Joined: 24 Jun 2008
Posts: 1849
Re: If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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2
kelvind13 wrote:
Is there a faster solution to this?

No, I think you've done this in the fastest possible way. It's clearly not a real GMAT question, so don't worry about how long it took - if you can solve it at all, you're doing very well.

There are a few real GMAT questions which test the prime factorizations of factorials, as this question does, but in every one of them, it is practical to work out the answer in two minutes without knowing any special formula. The real GMAT questions I've seen on this topic (there are three or four in the GMATFocus product, and one in the OG) are never more complicated than this one:

What is the largest integer k for which 5^k is a divisor of 50!

To answer that question without a formula, you only need to look at the ten multiples of five inside 50!, which can be done by hand within two minutes. I've never seen a real GMAT question in this format where you'd need to look at more than ten different multiples if you were solving by hand, so the question posted above is far more complicated than any real question I've encountered.
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Intern  Joined: 13 Mar 2011
Posts: 21
Re: If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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IanStewart wrote:
kelvind13 wrote:
Is there a faster solution to this?

No, I think you've done this in the fastest possible way. It's clearly not a real GMAT question, so don't worry about how long it took - if you can solve it at all, you're doing very well.

There are a few real GMAT questions which test the prime factorizations of factorials, as this question does, but in every one of them, it is practical to work out the answer in two minutes without knowing any special formula. The real GMAT questions I've seen on this topic (there are three or four in the GMATFocus product, and one in the OG) are never more complicated than this one:

What is the largest integer k for which 5^k is a divisor of 50!

To answer that question without a formula, you only need to look at the ten multiples of five inside 50!, which can be done by hand within two minutes. I've never seen a real GMAT question in this format where you'd need to look at more than ten different multiples if you were solving by hand, so the question posted above is far more complicated than any real question I've encountered.

Right now, the GMAT timer on 2 minutes 22 second to solve this problem correctly. I doubt too, that it is possible to solve this problem at this time.
Math Expert V
Joined: 02 Aug 2009
Posts: 8342
If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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1
1
leeto wrote:
IanStewart wrote:
kelvind13 wrote:
Is there a faster solution to this?

No, I think you've done this in the fastest possible way. It's clearly not a real GMAT question, so don't worry about how long it took - if you can solve it at all, you're doing very well.

There are a few real GMAT questions which test the prime factorizations of factorials, as this question does, but in every one of them, it is practical to work out the answer in two minutes without knowing any special formula. The real GMAT questions I've seen on this topic (there are three or four in the GMATFocus product, and one in the OG) are never more complicated than this one:

What is the largest integer k for which 5^k is a divisor of 50!

To answer that question without a formula, you only need to look at the ten multiples of five inside 50!, which can be done by hand within two minutes. I've never seen a real GMAT question in this format where you'd need to look at more than ten different multiples if you were solving by hand, so the question posted above is far more complicated than any real question I've encountered.

Right now, the GMAT timer on 2 minutes 22 second to solve this problem correctly. I doubt too, that it is possible to solve this problem at this time.

Hi,

just read this Q and few responses above..

I would have a point here..

We do not require to calculate anything here..
32 zeroes too is just a number which will not effect the answer..

A zero will be added when a 2 and a 5 is added...
every alternate term is even so we are worried only about a 5..

And 5 comes after every four number and two each will be odd and two even in these..
so there will be two ODD natural numbers before a 5 is added in the product and in turn an extra ZERO..

ans is 2..
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Re: If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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chetan2u wrote:

I would have a point here..

We do not require to calculate anything here..
32 zeroes too is just a number which will not effect the answer..

The '32' does matter here - the answer is not always '2'. You might try a few simpler questions to see why:

* For how many odd numbers n does n! have exactly 4 trailing zeros? (the answer is two, 21 and 23)

* For how many odd numbers n does n! have exactly 6 trailing zeros? (the answer is three, 25, 27, 29)

* For how many odd numbers n does n! have exactly 5 trailing zeros? (the answer is none)
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Math Expert V
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Posts: 8342
Re: If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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1
IanStewart wrote:
chetan2u wrote:

I would have a point here..

We do not require to calculate anything here..
32 zeroes too is just a number which will not effect the answer..

The '32' does matter here - the answer is not always '2'. You might try a few simpler questions to see why:

* For how many odd numbers n does n! have exactly 4 trailing zeros? (the answer is two, 21 and 23)

* For how many odd numbers n does n! have exactly 6 trailing zeros? (the answer is three, 25, 27, 29)

* For how many odd numbers n does n! have exactly 5 trailing zeros? (the answer is none)

hi,

I took somehow after crossing over to another 0, how many n odd natural numbers will have same trailing zeroes..
1) when a five is added due to addition of a multiple of 10.. x1 and x3 are the two..
2) when a five is added due to addition of a multiple of odd number.. x7 and x9 are the two..
3) in the case of
Quote:
* For how many odd numbers n does n! have exactly 5 trailing zeros? (the answer is none)

this happens because of addition of a number which is multiple of 5^2 or 5^3 and so on..
again as I took the case that we are xg over from x number of 0s to x+1 0s..

But I think read extra in the Q..
Thanks
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Re: If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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2
AKG1593 wrote:
If n is an odd natural number and n! ends with 32 zeros, then how many values of n are possible?

A. 2
B. 3
C. 1
D. 4
E. none

Spoiler: :: My doubt & OA
I think the answer should be B(135,137,139).But the OA is A.Can anyone explain?

We have geometric series:

$$\frac{n}{5} + \frac{n}{5^2} + \frac{n}{5^3} + … ≈ 32$$

$$(\frac{n}{5})/(1-\frac{1}{5}) ≈ 32$$

$$\frac{n}{4} ≈ 32$$

$$n ≈ 128$$

Nearest odd is 129, but 129! has only 31 trailing zero. In order to add additional zero we need to have additional multiple of 5 in factorial.
Hence, for 32 trailing zeros we have:

130!, 131!, 132!, 133! and 134! Starting from 135 factorial we’ll get additional multiple of 5 and additional trailing zero.

Picking up odd numbers we have – 131! and 133!

We need to remember:
Multiples of 5 add 1 traling zero,
- $$5^2$$ - 2 traling zeros
- $$5^3$$ - 3 traling zeros
- etc.
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Re: If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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Hi,

Dont need to go with formula.

We konw.. 32 zerores means.. 32 5's and 32 2's.. In any factorial of number, 2s is always be higher than 5's.

Now here Question is howmany odd numbers are possible...

means for any number of zeroes.. there will always 2 odd number..

starting for 1 to 5...1 and 3 are odd number.. whenever it hits 5 , we are adding one more zero to it.

Hope this logic helps.
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If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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pratik1709 wrote:
Hi,

Dont need to go with formula.

We konw.. 32 zerores means.. 32 5's and 32 2's.. In any factorial of number, 2s is always be higher than 5's.

Now here Question is howmany odd numbers are possible...

means for any number of zeroes.. there will always 2 odd number..

starting for 1 to 5...1 and 3 are odd number.. whenever it hits 5 , we are adding one more zero to it.

Hope this logic helps.

Hi pratik1709,

I think above logic might not work always.

if the question had asked number of possible values of odd number n such that 33 trailing zeroes.
then there are three such possible odd numbers, 135!, 137!, 139!

it depends on which parity of n adds that extra zero(es).

say odd number n adds extra zero(es), then there are 3 possible odd number, eg n - 135(odd) adds extra zero to previous 134!, then 135!, 137! and 139! are possible odd numbers of n such that n! has 33 trailing zeroes
say even number n adds extra zero(es), then there are 2 possible odd number.e.g n = 130(even) adds extra zero to previous 129!, then 131! and 133! are only possible numbers of n such that n! has 32 trailing zeroes
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Re: If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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Hello,

Is it true that the answer above has to be 2 or none? For any number of zeros, there will be 2 possible values of n, UNLESS that number is 5 taken to a power:

20! to 24! has four trailing zeros - 20/5 = 4 (so 21 and 23 would be possible answers for four zeros)
25! has six trailing zeros - 25/5 + 25/25 = 6

So no values of n! have five trailing zeros.
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Re: If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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AKG1593 wrote:
If n is an odd natural number and n! ends with 32 zeros, then how many values of n are possible?

A. 2
B. 3
C. 1
D. 4
E. none

n=odd, and n!=32 zeros = sum of the quotients of $$n/5+n/5^2+n/5^m=32…(where:5^m≤n)$$;
for n!=125! sum of the quotients is $$125/5+125/25+125/125=25+5+1=31$$
for n!=130! sum of the quotients is $$130/5+130/25+130/125=26+5+1=32$$
for n!=135! sum of the quotients is $$135/5+135/25+135/125=27+5+1=33$$
possible values of n are the odds from 130≤n<135 n={131,133}=2

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Re: If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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Bunuel wrote:
PathFinder007 wrote:
AKG1593 wrote:
If n is an odd natural number and n! ends with 32 zeros, then how many values of n are possible?

A. 2
B. 3
C. 1
D. 4
E. none

Spoiler: :: My doubt & OA
I think the answer should be B(135,137,139).But the OA is A.Can anyone explain?

Bunuel

i am not agree with option A as it is given as correct answer.

i am getting 135!, 137! and 139! for 32 trailing zeros. so i think it should be 3.

Thanks

131! has 32 trailing zeros: 131/5 + 131/25 + 131/125 = 26 + 5 + 1 = 32.
133! has 32 trailing zeros: 133/5 + 133/25 + 133/125 = 26 + 5 + 1 = 32.

But 135! has 33 trailing zeros: 135/5 + 135/25 + 135/125 = 27 + 5 + 1 = 33.

Hi! how do we reach upto those numbers? isn't there any method to solve this?
Math Expert V
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Posts: 60467
Re: If n is an odd natural number and n! ends with 32 zeros, the  [#permalink]

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priyatomar wrote:
Bunuel wrote:
PathFinder007 wrote:
i am not agree with option A as it is given as correct answer.

i am getting 135!, 137! and 139! for 32 trailing zeros. so i think it should be 3.

Thanks

131! has 32 trailing zeros: 131/5 + 131/25 + 131/125 = 26 + 5 + 1 = 32.
133! has 32 trailing zeros: 133/5 + 133/25 + 133/125 = 26 + 5 + 1 = 32.

But 135! has 33 trailing zeros: 135/5 + 135/25 + 135/125 = 27 + 5 + 1 = 33.

Hi! how do we reach upto those numbers? isn't there any method to solve this?

Links to the theory and similar questions are already given above. Here they ate again:

For similiar questions check Trailing Zeros Questions and Power of a number in a factorial questions in our Special Questions Directory.

_________________ Re: If n is an odd natural number and n! ends with 32 zeros, the   [#permalink] 10 Dec 2019, 00:11
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