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Manager  Joined: 14 May 2009
Posts: 186
Schools: Stanford, Harvard, Berkeley, INSEAD
If x is an integer, then how many digits does  [#permalink]

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Difficulty:

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Question Stats: 50% (00:57) correct 50% (00:24) wrong based on 30 sessions

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Enjoy.

Question : (If x is an integer, then how many digits does $$X^{101}$$ have?)

(1) $$X^{7}$$ has 15 digits.

(2) The units digits of $$X^{74}$$ is 1

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Hades
Manager  Joined: 14 May 2009
Posts: 186
Schools: Stanford, Harvard, Berkeley, INSEAD
Re: Tough DS  [#permalink]

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I think I made this question too hard _________________
Hades
Manager  Joined: 14 May 2009
Posts: 186
Schools: Stanford, Harvard, Berkeley, INSEAD
Re: Tough DS  [#permalink]

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Come on... don't be shy-- give it a shot _________________
Hades
SVP  Joined: 29 Aug 2007
Posts: 2310
Re: Tough DS  [#permalink]

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Hades wrote:
Enjoy.

Question : (If x is an integer, then how many digits does $$X^{101}$$ have?)

(1) $$X^{7}$$ has 15 digits.
(2) The units digits of $$X^{74}$$ is 1

Its not difficult rather time consuming and not a typical gmat question.

1. x could be any number between 100 and 138 (with the help of computer)
2. x could be any integer/number ending unit digit in 1 and 9 (not 3 and 7).

Togather, x could be either of 101, 109 or 111 or 119 or 121 or 129 or 131. They do not have same number of digits if each is raised to power 101. So E.
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Manager  Joined: 08 Feb 2009
Posts: 136
Schools: Anderson
Re: Tough DS  [#permalink]

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Hades wrote:
No calculators!

I would like to know the solution to this!

My guess is A.
SVP  Joined: 04 May 2006
Posts: 1605
Schools: CBS, Kellogg
Re: Tough DS  [#permalink]

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Hades wrote:
Enjoy.

Question : (If x is an integer, then how many digits does $$X^{101}$$ have?)

(1) $$X^{7}$$ has 15 digits.

(2) The units digits of $$X^{74}$$ is 1

This prob. helps me reach 801 score No clue _________________
SVP  Joined: 07 Nov 2007
Posts: 1607
Location: New York
Re: Tough DS  [#permalink]

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Hades wrote:
Enjoy.

Question : (If x is an integer, then how many digits does $$X^{101}$$ have?)

(1) $$X^{7}$$ has 15 digits.

(2) The units digits of $$X^{74}$$ is 1

(1)
100<=x< (1.d)*100 (where (1.d)^7 must be <10)

(1.d)^7 <10 ---> (1.d)^101 may be >10 --> which leads to multiple solutions.

not sufficient

(2)
last digit is either 1 or 9
not sufficient

combined.

x can be
101,109,111,121.. etc.
e.g
101= 1.01 *100 power 7 leads to 15 digits. --> power 101 leads N number of digits
111= 1.11 *100 power 7 leads to 15 digits.--> power 101 leads more than N number of digits

Will go with E.
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Manager  Joined: 15 Jan 2008
Posts: 241
Re: Tough DS  [#permalink]

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another E..

i would like to see an official explanation to this Question..
this is atypical to a regular gmat question..
Manager  Joined: 08 Feb 2009
Posts: 136
Schools: Anderson
Re: Tough DS  [#permalink]

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does it have anything to do with logarithms ?
Manager  Joined: 24 Jul 2009
Posts: 227
Re: Tough DS  [#permalink]

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Hades wrote:
Enjoy.

Question : (If x is an integer, then how many digits does $$X^{101}$$ have?)

(1) $$X^{7}$$ has 15 digits.

(2) The units digits of $$X^{74}$$ is 1

From Question Stem:
Let k = x^101.
log (k) = 101 log (x).

From Statement 1: let P= x^7.
log (p) = 7 log (x).

Now given log(P) = 15, So log(x) = 15/7.
Insert the value of log (x) in the question stem. we get k= 101* 15/7. The value of k is the no. of digits in x^101

So A is sufficient.

Please comment guys..!!
GMAT Tutor G
Joined: 24 Jun 2008
Posts: 1530
Re: Tough DS  [#permalink]

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nverma wrote:
Hades wrote:
Enjoy.

Question : (If x is an integer, then how many digits does $$X^{101}$$ have?)

(1) $$X^{7}$$ has 15 digits.

(2) The units digits of $$X^{74}$$ is 1

From Question Stem:
Let k = x^101.
log (k) = 101 log (x).

From Statement 1: let P= x^7.
log (p) = 7 log (x).

Now given log(P) = 15, So log(x) = 15/7.
Insert the value of log (x) in the question stem. we get k= 101* 15/7. The value of k is the no. of digits in x^101

So A is sufficient.

Please comment guys..!!

Unfortunately that doesn't quite work. If we're working in base 10, and p has fifteen digits, then that means that 10^14 < p < 10^15. In other words, that means that 14 < log(p) < 15. There's no way to find an exact value for log(p) here without more information.

(and so as not to cause any panic among test takers, logarithms are definitely *not* tested on the GMAT!)
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Manager  Joined: 24 Jul 2009
Posts: 227
Re: Tough DS  [#permalink]

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IanStewart wrote:
nverma wrote:
Hades wrote:
Enjoy.

Question : (If x is an integer, then how many digits does $$X^{101}$$ have?)

(1) $$X^{7}$$ has 15 digits.

(2) The units digits of $$X^{74}$$ is 1

From Question Stem:
Let k = x^101.
log (k) = 101 log (x).

From Statement 1: let P= x^7.
log (p) = 7 log (x).

Now given log(P) = 15, So log(x) = 15/7.
Insert the value of log (x) in the question stem. we get k= 101* 15/7. The value of k is the no. of digits in x^101

So A is sufficient.

Please comment guys..!!

Unfortunately that doesn't quite work. If we're working in base 10, and p has fifteen digits, then that means that 10^14 < p < 10^15. In other words, that means that 14 < log(p) < 15. There's no way to find an exact value for log(p) here without more information.

(and so as not to cause any panic among test takers, logarithms are definitely *not* tested on the GMAT!)

Hello Ian Stewart

I agree that we don't get any exact integer value of log(p). But just for knowledge can you tell whether the approach was right.
GMAT Tutor G
Joined: 24 Jun 2008
Posts: 1530
Re: Tough DS  [#permalink]

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nverma wrote:
From Question Stem:
Let k = x^101.
log (k) = 101 log (x).

From Statement 1: let P= x^7.
log (p) = 7 log (x).

Now given log(P) = 15, So log(x) = 15/7.
Insert the value of log (x) in the question stem. we get k= 101* 15/7. The value of k is the no. of digits in x^101

So A is sufficient.

________

Hello Ian Stewart

I agree that we don't get any exact integer value of log(p). But just for knowledge can you tell whether the approach was right.

Yes, you can use your method to determine precisely what Statement 1 tells us, provided that you use the correct inequalities - it's probably the best approach here, at least without a calculator (please note that this is *not* tested on the GMAT - for interest only!).

14 < log(x^7) < 15
14 < 7*log(x) < 15
2 < log(x) < 15/7
202 < 101*log(x) < (101)(15)/7
202 < log(x^101) < 216.428....

from which you can see that the number of digits in x^101 could range from 203 to 217.
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Manager  Joined: 16 Apr 2009
Posts: 245
Re: Tough DS  [#permalink]

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what is the OA and OE for this ? is this a gmat question _________________
Always tag your question
GMAT Tutor G
Joined: 24 Jun 2008
Posts: 1530
Re: Tough DS  [#permalink]

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ichha148 wrote:
what is the OA and OE for this ? is this a gmat question I take it, from the comments above, that this is a question designed by one of the members here. It is not a real GMAT question, and you won't see something quite like this on test day - I suppose there's a small chance you could see a much simpler question testing the same concepts, but not with numbers nearly as large.
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Manager  Joined: 11 Sep 2009
Posts: 129
Re: Tough DS  [#permalink]

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I believe the answer is E as well... my technique is not nearly as refined as the one posted by Ian above though:

Statement 2: The Units digit of $$X^{74}$$ is 1.

i.e X is an integer with a units digit of 1 or 9. Not sufficient.

Statement 1: $$X^7$$ has 15 digits.

$$10^{14} < X^7 < 10^{15}$$

$$100 < X < 100*r$$

...where r = $$\sqrt{10}$$

Also, since we know X is greater than or equal to 100, and we are taking X^101, we need to know the EXACT value of X in order to determine how many digits X^101 has since:

$$\frac{101^{101}}{100^{101}} = 1.01^{101} > 10$$

i.e. X^101 and (X+1)^{101} will always differ by a factor of at least 10, meaning they will never have the same amount of digits.

Now here's where it gets kind of messy:

I know 1.3^7 = 1.69 * 1.69 * 1.69 * 1.3 < 1.7^3 * 1.3 < 10
So I know that the upper bound is AT LEAST 130 on X.

Therefore, insufficient.

Evaluating Both Statements Together:

Statement 2: X has units digit of 1 or 9
Statement 1: 100 <= X <= 130 (conservative estimate), must know X to exact number

101, 109, 111, 119... etc. there are multiple numbers that satisfy both cases and we know that they will all have different amount of digits. Therefore, insufficient.

Therefore, the correct answer is E. Sorry if this is confusing to some, I don't know an elegant solution to this problem other than the logarithmic one produced above.
Manager  Joined: 29 Dec 2009
Posts: 59
Location: india
Re: Tough DS  [#permalink]

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ans shld be e coz
100^7 = 10^14 so from here numbering of 15 digits starts ... but it could be 101, 109, 111 all have 15 digits
b statement nothing helps ... so ans e
Manager  Status: Can't give up
Joined: 20 Dec 2009
Posts: 211
Re: Tough DS  [#permalink]

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another E....

1.) x can be any value same goes with 2)
Manager  Joined: 13 Dec 2009
Posts: 219
Re: Tough DS  [#permalink]

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It should be E cannot deduce no of digits from both the stmts.
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Intern  B
Joined: 28 Nov 2014
Posts: 10
Re: If x is an integer, then how many digits does  [#permalink]

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A quant expert, who solves this kind of question for gmat, can score < 600.

--== Message from the GMAT Club Team ==--

THERE IS LIKELY A BETTER DISCUSSION OF THIS EXACT QUESTION.
This discussion does not meet community quality standards. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you. Re: If x is an integer, then how many digits does   [#permalink] 10 Aug 2017, 08:12
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# If x is an integer, then how many digits does

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