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# For each positive integer n, p(n) is defined to be the product of..

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Intern
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For each positive integer n, p(n) is defined to be the product of.. [#permalink]

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05 May 2016, 09:16
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Difficulty:

55% (hard)

Question Stats:

64% (00:51) correct 36% (00:59) wrong based on 397 sessions

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For each positive integer $$n$$, $$p(n)$$ is defined to be the product of the digits of $$n$$. For example, $$p(724) = 56$$ since $$7 * 2 * 4 =56$$.

Which of the following statements must be true?

I. $$p(10n) = p(n)$$

II. $$p(n+1) > p(n)$$

III. $$p(2n) = 2p(n)$$

--

A. None
B. I and II only
C. I and III only
D. II and III only
E. I, II, and III
[Reveal] Spoiler: OA

Last edited by Vyshak on 05 May 2016, 09:39, edited 1 time in total.
Formatted the question

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Re: For each positive integer n, p(n) is defined to be the product of.. [#permalink]

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05 May 2016, 09:37
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I) p(10n) = p(n)
p(10n) will always have units digit as 0 --> p(10n) = 0
p(n) can be any integer.
Not a must be true statement

II) p(n + 1) > p(n)
If n = 2; p(n +1) = 3 and p(n) = 2 --> p(n + 1) > p(n)
If n = 9; p(n + 1) = 0 and p(n) = 9 --> p(n + 1) < p(n)
Not a must be true statement

III) p(2n) = 2*p(n)
If n = 12; p(2n) = p(24) = 8 and 2*p(n) = 4
Not a must be true statement

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Magoosh GMAT Instructor
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Re: For each positive integer n, p(n) is defined to be the product of the [#permalink]

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17 Jun 2016, 15:47
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Expert's post
For each positive integer n, p(n) is defined to be the product of the digits of n. For example, p(724) = 56, since 7 x 2 x 4 = 56. Which of the following statements must be true?

I. p(10n) = p(n)
II. p(n + 1) > p(n)
III. p(2n) = 2p(n)

A: None
B: I and II only
C: I and III only
D: II and III only
E: I, II and III

I'm happy to respond.

My friend, I gather that you are relatively new to GMAT Club. I will share with you an important piece of GC etiquette. Please do NOT start a brand new thread for a question that has already been posted. This particular question has already be posted here:
for-each-positive-integer-n-p-n-is-defined-to-be-the-product-of-217887.html
Always search before you start a separate post. You may find your question answered in that post, and if you don't, you can add your own question to that existing thread. Bunuel will merge this post into that thread.

Does all this make sense?
Mike
_________________

Mike McGarry
Magoosh Test Prep

Education is not the filling of a pail, but the lighting of a fire. — William Butler Yeats (1865 – 1939)

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Re: For each positive integer n, p(n) is defined to be the product of.. [#permalink]

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29 Aug 2016, 13:08
Bunuel wrote:
For each positive integer n, p(n) is defined to be the product of the digits of n. For example, p(724) = 56, since 7 x 2 x 4 = 56. Which of the following statements must be true?

I. p(10n) = p(n)
II. p(n + 1) > p(n)
III. p(2n) = 2p(n)

A: None
B: I and II only
C: I and III only
D: II and III only
E: I, II and III

___________________________
Merging topics.

Read the response by Vyshak and still confused. Didn't see other responses so reaching out.

Thanks!

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Re: For each positive integer n, p(n) is defined to be the product of.. [#permalink]

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30 Aug 2016, 15:40
8
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Expert's post
5
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BOOKMARKED
broilerc wrote:
For each positive integer $$n$$, $$p(n)$$ is defined to be the product of the digits of $$n$$. For example, $$p(724) = 56$$ since $$7 * 2 * 4 =56$$.

Which of the following statements must be true?

I. $$p(10n) = p(n)$$

II. $$p(n+1) > p(n)$$

III. $$p(2n) = 2p(n)$$

--

A. None
B. I and II only
C. I and III only
D. II and III only
E. I, II, and III

Let’s go through each statement given in the Roman numerals.

I. p(10n) = p(n)

This is not true. For example, if n = 12, p(12) = 2 since 1 x 2 = 2. However, 10n = 10(12) = 120 and p(120) = 0 since 1 x 2 x 0 = 0. Since p(120) ≠ p(12), p(10n) = p(n) is not a true statement.

II. p(n +1) > p(n)

This is not true. For example, if n = 19, then p(19) = 9 since 1 x 9 = 9. However, n + 1 = 19 + 1 = 20 and p(20) = 0 since 2 x 0 = 0. Since p(20) < p (19), p(n +1) > p(n) is not a true statement.

Since neither I nor II is true, it can’t be choices B, C, D or E. So the correct choice must be A. However, let’s show III is also not true.

III. p(2n) = 2p(n)

For example, if n = 15, then p(15) = 5 since 1 x 5 = 5 and 2p(15) = 2 x 5 = 10. However, 2n = 2 x 15 = 30 and p(30) = 0 since 3 x 0 = 0. Since p(30) ≠ 2p(15), p(2n) = 2p(n) is not a true statement.

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Re: For each positive integer n, p(n) is defined to be the product of.. [#permalink]

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31 Aug 2016, 03:48
JeffTargetTestPrep wrote:
broilerc wrote:
For each positive integer $$n$$, $$p(n)$$ is defined to be the product of the digits of $$n$$. For example, $$p(724) = 56$$ since $$7 * 2 * 4 =56$$.

Which of the following statements must be true?

I. $$p(10n) = p(n)$$

II. $$p(n+1) > p(n)$$

III. $$p(2n) = 2p(n)$$

--

A. None
B. I and II only
C. I and III only
D. II and III only
E. I, II, and III

Let’s go through each statement given in the Roman numerals.

I. p(10n) = p(n)

This is not true. For example, if n = 12, p(12) = 2 since 1 x 2 = 2. However, 10n = 10(12) = 120 and p(120) = 0 since 1 x 2 x 0 = 0. Since p(120) ≠ p(12), p(10n) = p(n) is not a true statement.

II. p(n +1) > p(n)

This is not true. For example, if n = 19, then p(19) = 9 since 1 x 9 = 9. However, n + 1 = 19 + 1 = 20 and p(20) = 0 since 2 x 0 = 0. Since p(20) < p (19), p(n +1) > p(n) is not a true statement.

Since neither I nor II is true, it can’t be choices B, C, D or E. So the correct choice must be A. However, let’s show III is also not true.

III. p(2n) = 2p(n)

For example, if n = 15, then p(15) = 5 since 1 x 5 = 5 and 2p(15) = 2 x 5 = 10. However, 2n = 2 x 15 = 30 and p(30) = 0 since 3 x 0 = 0. Since p(30) ≠ 2p(15), p(2n) = 2p(n) is not a true statement.

Thanks Jeff! Clear now.
Btw, did you think to pick specifically those numbers to test? I see you tried to pick ones were you would end up with a 0 as a multiplier.

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For each positive integer n, p(n) is defined to be the product of.. [#permalink]

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31 Aug 2016, 03:54
18967mba wrote:
JeffTargetTestPrep wrote:
broilerc wrote:
For each positive integer $$n$$, $$p(n)$$ is defined to be the product of the digits of $$n$$. For example, $$p(724) = 56$$ since $$7 * 2 * 4 =56$$.

Which of the following statements must be true?

I. $$p(10n) = p(n)$$

II. $$p(n+1) > p(n)$$

III. $$p(2n) = 2p(n)$$

--

A. None
B. I and II only
C. I and III only
D. II and III only
E. I, II, and III

Let’s go through each statement given in the Roman numerals.

I. p(10n) = p(n)

This is not true. For example, if n = 12, p(12) = 2 since 1 x 2 = 2. However, 10n = 10(12) = 120 and p(120) = 0 since 1 x 2 x 0 = 0. Since p(120) ≠ p(12), p(10n) = p(n) is not a true statement.

II. p(n +1) > p(n)

This is not true. For example, if n = 19, then p(19) = 9 since 1 x 9 = 9. However, n + 1 = 19 + 1 = 20 and p(20) = 0 since 2 x 0 = 0. Since p(20) < p (19), p(n +1) > p(n) is not a true statement.

Since neither I nor II is true, it can’t be choices B, C, D or E. So the correct choice must be A. However, let’s show III is also not true.

III. p(2n) = 2p(n)

For example, if n = 15, then p(15) = 5 since 1 x 5 = 5 and 2p(15) = 2 x 5 = 10. However, 2n = 2 x 15 = 30 and p(30) = 0 since 3 x 0 = 0. Since p(30) ≠ 2p(15), p(2n) = 2p(n) is not a true statement.

Thanks Jeff! Clear now.
Btw, did you think to pick specifically those numbers to test? I see you tried to pick ones were you would end up with a 0 as a multiplier.

Yeah, so I thought it would be easiest to select numbers that had a zero as one of the digits. Glad I could help!
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500+ lessons 3000+ practice problems 800+ HD solutions

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Re: For each positive integer n, p(n) is defined to be the product of.. [#permalink]

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21 Oct 2016, 10:22
broilerc wrote:
For each positive integer $$n$$, $$p(n)$$ is defined to be the product of the digits of $$n$$. For example, $$p(724) = 56$$ since $$7 * 2 * 4 =56$$.

Which of the following statements must be true?

I. $$p(10n) = p(n)$$

II. $$p(n+1) > p(n)$$

III. $$p(2n) = 2p(n)$$

--

A. None
B. I and II only
C. I and III only
D. II and III only
E. I, II, and III

let n=23
Consider III
P(2n)=p(46)=4*6=24
p(n)=2*3=6
24 is not equal to two times of 6.........so III is not must be true ruling out C, D and E options.

Come to I, any number multiplied by ten will have 0 as one of its integers and product will be zero. Ruling out B as well leaving only A as correct choice.

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Re: For each positive integer n, p(n) is defined to be the product of.. [#permalink]

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29 Oct 2017, 06:11
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