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If n is a positive integer, is 10^n - 1 divisible by q?

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If n is a positive integer, is 10^n - 1 divisible by q? [#permalink] New post 17 Jun 2011, 22:27
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E

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58% (01:00) correct 42% (01:14) wrong based on 71 sessions
If n is a positive integer, is 10^n - 1 divisible by q?

(1) q is not divisible by either 5 or 2
(2) q is not divisible by 9

My query :
[Reveal] Spoiler:
From statement 1-Q can be either 3,7,9. insufficient.
From statement 2-Q can be either 3,7 insufficient(cannot be 9).

combined we know Q can be either 3 or 7, hence answer to this question is C-NO.
[Reveal] Spoiler: OA

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Last edited by Bunuel on 12 Oct 2013, 08:28, edited 1 time in total.
Renamed the topic, edited the question and added the OA.
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Re: m12,#2 [#permalink] New post 18 Jun 2011, 00:25
bblast wrote:
could not find this discussed anywhere :

If n is a positive integer, is (10^n)-1 divisible by Q ?

Q is not divisible by either 5 or 2.
Q is not divisible by 9.

OA is
[Reveal] Spoiler:
E
.


My query :

From statement 1-Q can be either 3,7,9. insufficient.
From statement 2-Q can be either 3,7 insufficient(cannot be 9).

combined we know Q can be either 3 or 7, hence answer to this question is C-NO.


Not sure how you got C .
Im getting E
st.1 Q is not divisible by 5 or 2
so Q can be 3,7,9,13,17,.............. not sufficient

st2: Q is not divisible by 9
Q can be 2,3,4,5,6,7,8,13,17......... not sufficient

1 and 2 together

3,7,13,17....

10^n-1 is divisible by 3 but not by 7
hence insufficient . E

Note: where is question its mentioned that Q is a single digit Integer? U can take any number
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Re: m12,#2 [#permalink] New post 18 Jun 2011, 00:28
Oops:

I need to hammer my skull to get things aligned. :oops:

the answer is indeed E.
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Re: If n is a positive integer, is (10^n)-1 divisible by Q ? [#permalink] New post 12 Oct 2013, 06:14
bblast wrote:
could not find this discussed anywhere :

If n is a positive integer, is (10^n)-1 divisible by Q ?

Q is not divisible by either 5 or 2.
Q is not divisible by 9.

OA is
[Reveal] Spoiler:
E
.


My query :

From statement 1-Q can be either 3,7,9. insufficient.
From statement 2-Q can be either 3,7 insufficient(cannot be 9).

combined we know Q can be either 3 or 7, hence answer to this question is C-NO.


Easier method.

10^n - 1 / q an integer?

So let's say n=1, then we have 9 / q an integer?

Statement 1:

Q not div by 5 or 2, well if q=3 then yes, but if q = 7 the no.
Insuff

Statement 2: q not div by 9

If q = 1 then yes, q = 7 then no
Insuff

Both together

q = 1 then yes, q=7 then no

Answer is E

Hope its clear
Cheers
J

Last edited by jlgdr on 08 Feb 2014, 06:40, edited 1 time in total.
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Re: If n is a positive integer, is (10^n)-1 divisible by Q ? [#permalink] New post 12 Oct 2013, 08:31
Expert's post
jlgdr wrote:
bblast wrote:
If n is a positive integer, is 10^n - 1 divisible by q?

(1) q is not divisible by either 5 or 2
(2) q is not divisible by 9


I must have gotten this one wrong, but here is my reasoning.

Statement 1:

Q is not divisible by either 2 or 5 says that 10n is not a multiple of q. But we don't know if 10^n-1 could be because 1 of course is not a multiple of 10. Remember when we If you add (or subtract) two multiples of N, the result is always a multiple of N but If you add two non-multiples of N, the result could be either a multiple of N or a non-multiple of N. So in this case, we don't know

Further more as n has to be a postiive integer than 10 is at least 10, so our possible values are 9,99,999 etc...

Statement 2:
If q is not divisible by 9 then IMO 10^n-1 is not divisible by 9 for the reason mentioned above.

Hence IMO answer should be (B)

Experts please advice
Cheers


Say n=1. Then 10^n-1=9.

Now, if q=1, then 9 IS divisible by q but if q=7, then 9 is NOT divisible by q.

Answer: E.

Hope it's clear.
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Re: If n is a positive integer, is (10^n)-1 divisible by Q ?   [#permalink] 12 Oct 2013, 08:31
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