Number Properties

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Number Properties

by Aman verma » Fri Feb 05, 2010 7:52 am
Q: When a number is divided by 1,2,3,4,5,.........(n-1),n individually it leaves 0,1,2,3,4,5,.......(n-2),(n-1) respective reminders, then this number can be :

(i) n! (ii) n! - 1 (iii) [ (L.C.M of 1,2,3,4,5,........n) - 1 ]


a) Both (i) and (ii)

b) Both (ii) and (iii)

c) Only (iii)

d) Only (ii)

e) Can't be determined


Can anybody help me on this ?
Source: — Problem Solving |

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by thephoenix » Fri Feb 05, 2010 8:11 am
IMO D

i tried with sme no...

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by Aman verma » Fri Feb 05, 2010 11:23 am
I guess this is a 400 range question ! What do you people think ? Any guess . I will come up with the solution soon but I would like to hear from anybody who has an ingenious approach to solving this problem.

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by harshavardhanc » Fri Feb 05, 2010 11:29 am
Case 1:

One thing to be noted here is when the number is divided by 2 : 1 is the remainder

=> the number has to be odd.

now, we know that every factorial (except 0! and 1!) is even, hence (i) cannot be the number.

Also, when you are taking an LCM of any number above with 2, it will be even and subtracting 1 from the LCM will give us an odd number. So, (ii) and (iii) can be the numbers.

Case 2:

But, it is never mentioned in the question that you cannot have 1 as the number. In that case, I can view 1 as 1! (n!) and (i) also becomes correct.


Therefore IMO the number can't be determined.

what's the OA and source?
Last edited by harshavardhanc on Fri Feb 05, 2010 1:07 pm, edited 1 time in total.
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by ajith » Fri Feb 05, 2010 11:42 am
Aman verma wrote:Q: When a number is divided by 1,2,3,4,5,.........(n-1),n individually it leaves 0,1,2,3,4,5,.......(n-2),(n-1) respective reminders, then this number can be :

(i) n! (ii) n! - 1 (iii) [ (L.C.M of 1,2,3,4,5,........n) - 1 ]


a) Both (i) and (ii)

b) Both (ii) and (iii)

c) Only (iii)

d) Only (ii)

e) Can't be determined


Can anybody help me on this ?
When n =1 the number is 1 (n!)
When n=2 the number is 3
when n =3 the number is 5 (n! -1)
when n=4 the number is 15
When n =5 the number is 15

I vote for A
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by harshavardhanc » Fri Feb 05, 2010 12:38 pm
Aman verma wrote: When n =1 the number is 1 (n!)
When n=2 the number is 3
when n =3 the number is 5 (n! -1)
when n=4 the number is 15
When n =5 the number is 15

I vote for A
Did not get what number are you trying to divide in each case ?
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by ajith » Fri Feb 05, 2010 12:47 pm
harshavardhanc wrote: When n =1 the number is 1 (n!)
1 divided 1 leaves a remainder of 0

When n=2 the number is 3
3 divided by 1 leaves a remainder 0
3 divided by 2 leaves a remainder 1

when n =3 the number is 5 (n! -1) (LCM of 1,2,3 =6 6-1 =5)
5 divided by 1 leaves a remainder 0
5 divided by 2 leaves a remainder 1
5 divided by 3 leaves a remainder 2

when n=4 the number is 11
11 divided by 1 leaves a remainder of 0
11 divided by 2 leaves a remainder of 1
11 divided by 3 leaves a remainder of 2
11 divided by 4 leaves a remainder of 3

LCM of 1,2,3,4 =12 = 12-1 =11

I quite do not know 1! counts
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by harshavardhanc » Fri Feb 05, 2010 1:04 pm
ajith wrote: When n =1 the number is 1 (n!)
1 divided 1 leaves a remainder of 0

When n=2 the number is 3
3 divided by 1 leaves a remainder 0
3 divided by 2 leaves a remainder 1

when n =3 the number is 5 (n! -1) (LCM of 1,2,3 =6 6-1 =5)
5 divided by 1 leaves a remainder 0
5 divided by 2 leaves a remainder 1
5 divided by 3 leaves a remainder 2

when n=4 the number is 11
11 divided by 1 leaves a remainder of 0
11 divided by 2 leaves a remainder of 1
11 divided by 3 leaves a remainder of 2
11 divided by 4 leaves a remainder of 3

LCM of 1,2,3,4 =12 = 12-1 =11

I quite do not know 1! counts


Ok. so you now think the answer is "can't be determined". Good. have now got one more supporter.

what you did was the " hit and try" method which works fine if the limit of n is small. You were lucky to find 3, 5 and 11/

In this case, no information is given about n, so you need to find a generic approach to solve and a proper logic to proceed in cases like these.

really curious to see the OA and the explanation.
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by ajith » Fri Feb 05, 2010 1:19 pm
harshavardhanc wrote:
Ok. so you now think the answer is "can't be determined". Good. have now got one more supporter.

When did I say I support that view - I would rather go with B since I am sure that II and III have examples

what you did was the " hit and try" method which works fine if the limit of n is small. You were lucky to find 3, 5 and 11

What I did was hit and try and thanks for educating me that I was lucky and I did trial and error - much appreciated


In this case, no information is given about n, so you need to find a generic approach to solve and a proper logic to proceed in cases like these.

I agree with you

really curious to see the OA and the explanation.
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by harshavardhanc » Fri Feb 05, 2010 1:58 pm
ajith wrote:
harshavardhanc wrote:
Ok. so you now think the answer is "can't be determined". Good. have now got one more supporter.

When did I say I support that view - I would rather go with B since I am sure that II and III have examples

what you did was the " hit and try" method which works fine if the limit of n is small. You were lucky to find 3, 5 and 11

What I did was hit and try and thanks for educating me that I was lucky and I did trial and error - much appreciated


In this case, no information is given about n, so you need to find a generic approach to solve and a proper logic to proceed in cases like these.

I agree with you

really curious to see the OA and the explanation.
wow!!! so colorful!! but would hardly impress anyone. it would have been better if you would have spent ur time in finding an approach.

Now coming to your explanation :

If you have found out that n! , n! -1 and [ (L.C.M of 1,2,3,4,5,........n) - 1 ] each have their example, there is no such option as (i) or (ii) or (iii) in the answer choices, and you are still marking B, then you are at risk buddy! ;) take care.

anyway, let's see what the OA is.
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by ajith » Fri Feb 05, 2010 9:24 pm
harshavardhanc wrote: wow!!! so colorful!! but would hardly impress anyone. it would have been better if you would have spent ur time in finding an approach.
Well, There is something called Chinese remainder theorem and if you want to break your head answering this question you should check it out. I am not doing it myself because I am pretty sure that it will not be asked in GMAT and I am ready to run that risk.

About the quote, Please mind your own business
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by Aman verma » Sat Feb 06, 2010 1:42 am
Only few people attempted this question.

Harshavardhan said : it is never mentioned in the question that you cannot have 1 as the number. Now the number cannot possibly be 1 because if you divide 1 by any integer other than 1 you will always have 1 as the remainder.
Also when Harshavardha mentions even or odd ,he cannot be more wrong because the question has got nothing to do with even or odd. At best Harshavardhan's attempt at this question be called a conjecture which is not bassed on sound mathematical principles.

Now to the solution: the least possible number is = [L.C.M of 1,2,3,4.........n] -1.
Now if L.C.M of ( 1,2,3,4........n)= L ,then the next higher number=(m.L) -1 and m.L = n! for a suitable value of m; m belonging to Natural Numbers. Hence B is the most appropriate answer.

The most important fundamental property to note in this question is m.L = n! .Also please notice that the remainder is always 1 less than the divisor which is at the crux of the problem.
Last edited by Aman verma on Sat Feb 06, 2010 10:49 am, edited 1 time in total.

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by harshavardhanc » Sat Feb 06, 2010 4:16 am
Aman verma wrote:Only few people attempted this question.

Harshavardhan said : it is never mentioned in the question that you cannot have 1 as the number. Now the number cannot possibly be 1 because if you divide 1 by any integer other that 1 and 0 you will always have 1 as the remainder.
Also when Harshavardha mentions even or odd ,he cannot be more wrong because the question has got nothing to do with even or odd. At best Harshavardhan's attempt at this question be called a conjecture which is not bassed on sound mathematical principles.
you've called for it.. ..here's my reply to few of the points that you've raised :

First, whenever you are posting a question on a public forum, asking others to respond, please make sure that you replicate the exact question, without adding your own "touch" . I'm sure that you've paraphrased the question and have not copy-pasted it because :
respective reminders,
for your information, it's always a remainder which is left when you perform division. "Reminder" is what you use when you don't trust your memory. Not your fault, it's the way you've been "trained".

Moreover the correct options IMO would say :

A) Either (i) or (ii)
B) Either (ii) or (iii)
.
.

A number cannot be n! AND n! -1 at the same time.

Now, as you have done these types of mistake at the first place, I cannot be more sure that there is some additional information in the question, which in your paraphrasing, has been omitted.

Now the number cannot possibly be 1 because if you divide 1 by any integer other that 1 and 0 you will always have 1 as the remainder.
Buddy, please read the question ( that you have written ;) ) very very carefully.

It says, when you are dividing a number by 1, 2,3 .... n , it gives remainders as 0,1,... (n-1).

The thing to understand here is you are only dividing the number by 1 -> n .

Now when you have 1! as the number, n becomes 1 (n!) and hence, you have to divide 1! by 1 -> 1 ( i.e only by one number) and can you tell me what will be the remainder ? come on... you can do this calculation !!! remainder = 0 , which is (n-1).

Got it?! This satisfies the condition which YOU have mentioned.
which is not bassed on sound mathematical principles.
ha ha ha!! can't stop laughing !!! please.......

I'll quote another of your written sentences, and tell you a demo of sound mathematical principles :
because if you divide 1 by any integer other that 1 and 0
divide by zero?????????? what a sound mathematical principle!!!!!
now you too have joined me in the laughter riot!!!??? haven't you???

As far as I can see your explanation, I can only view it as a mugged-up explanation in the source from where you have tried to take this problem, reason being if you have genuinely tried to solve a mathematical problem ( mind you, it is different from practicing maths problem, which I am sure you have done in your school/engg) , you would have not said this :
the question has got nothing to do with even or odd
If you have a mathematical brain, you will know that there are multiple ways to solve a problem. Here as well, if you observe every value of 'n' will be odd. It cannot be even, reason being, the conditions put in the questions. Don't put stress on your brain, because it is said "it is better to avoid an activity where there is resistance".

In the end, I am not denying that the OA is B . It's just that I definitely think that some vital point, present in the original question, has been missed while replicating the question here.

Let's see if Mods/instructors can pitch in with their ideas.
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by Aman verma » Sat Feb 06, 2010 9:22 am
Well hello,

First of all I am not an engineering student. Secondly the question and the solution is written as it is , it is literally a verbatim. Now 1 cannot possibly be the number because any number is divisible by 1 and will leave remainder 0 but it will never leave remainder 2,3 ,4......when divided by 3,4,5........if the number is 1. And as regards Harsha's higly appreciated mathematical brain then he should have considered not only even odd but other million ways of solving the problem and wasted valuable time . Also when I mentioned about dividing 1 by zero all I meant to say is that the answer will be undefined and one will always get a remainder 1 if 1 is divided by any integer other than 1. Now as we are talking about integers one may consider 0 as the divisor mistakenly.To avoid such an error I referred to zero. I would have ignored zero in the first place but for the sake of completeness of the question zero was referred. Moreover Harsha don't have to worry about my brain as their is neither any resistance nor any stress, he better be bothering about his own highly appreciated mathematical brain. Regarding the 'reminder ' part of the question it was only a typing error, which even anybody even without brain would recognise a typing error and not intentional nor an attempt at modifying the question. Regarding trusting memory I don't need reminders . Also regarding my training Harsha can't even imagine cause he could never have gone through such rigours but that is a different matter has got nothing to do with GMAT math.
Harsha should better care about the lousy training he himself has gone through.

After all the talks,I would mention I never meant to offend anybody in the first place. However if anybody is hurt ,he should have mentioned that instead of using such lousy language. Also I never forced anybody to provide answers and I just pointed out some flaws in the answers given which I think can also occur in my answers. Anyway nobody is forced to answer to give an answer and if anybody uses such language I promise I will take that person in hand and give such reply that he or she cannot imagine in his / her seven lives,no offence !!
Last edited by Aman verma on Sat Feb 06, 2010 10:52 pm, edited 1 time in total.

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by harshavardhanc » Sat Feb 06, 2010 9:31 am
Aman verma wrote: Anyway nobody is forced to answer to give an answer and if anybody uses such language I promise I will take that person in hand and give such reply that he or she cannot imagine in his / her seven lives,no offence !!
Ooooooooooooh!!!!!!!!!!!!! I'm really scared!!!!! ;)

well, I will say : you can just try!
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