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Intern  Joined: 17 Jan 2016
Posts: 10
For each positive integer n, p(n) is defined to be the product of..  [#permalink]

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Difficulty:   55% (hard)

Question Stats: 63% (01:51) correct 37% (01:53) wrong based on 559 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

Originally posted by broilerc on 05 May 2016, 10:16.
Last edited by Vyshak on 05 May 2016, 10:39, edited 1 time in total.
Formatted the question
Target Test Prep Representative G
Status: Head GMAT Instructor
Affiliations: Target Test Prep
Joined: 04 Mar 2011
Posts: 2815
Re: For each positive integer n, p(n) is defined to be the product of..  [#permalink]

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13
10
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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Jeff@TargetTestPrep.com

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Marshall & McDonough Moderator D
Joined: 13 Apr 2015
Posts: 1684
Location: India
Re: For each positive integer n, p(n) is defined to be the product of..  [#permalink]

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10
6
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

General Discussion
Magoosh GMAT Instructor G
Joined: 28 Dec 2011
Posts: 4472
Re: For each positive integer n, p(n) is defined to be the product of the  [#permalink]

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1
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)
Current Student Joined: 21 Apr 2016
Posts: 28
Location: United States
Re: For each positive integer n, p(n) is defined to be the product of..  [#permalink]

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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!
Current Student Joined: 21 Apr 2016
Posts: 28
Location: United States
Re: For each positive integer n, p(n) is defined to be the product of..  [#permalink]

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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.
Target Test Prep Representative G
Status: Head GMAT Instructor
Affiliations: Target Test Prep
Joined: 04 Mar 2011
Posts: 2815
For each positive integer n, p(n) is defined to be the product of..  [#permalink]

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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!
_________________

Jeffrey Miller

Head of GMAT Instruction

Jeff@TargetTestPrep.com

See why Target Test Prep is the top rated GMAT quant course on GMAT Club. Read Our Reviews

If you find one of my posts helpful, please take a moment to click on the "Kudos" button.

Retired Moderator S
Joined: 18 Sep 2014
Posts: 1093
Location: India
Re: For each positive integer n, p(n) is defined to be the product of..  [#permalink]

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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.
Intern  Joined: 08 Mar 2017
Posts: 1
Re: For each positive integer n, p(n) is defined to be the product of..  [#permalink]

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Hello,

They mentionned for any positive number n the multiplication works. so p(10n)=1*n without multiplying it by 0 because 0 is not considered positive.
What is the part I don't get?

Best,
G
Intern  B
Joined: 30 Nov 2017
Posts: 23
Re: For each positive integer n, p(n) is defined to be the product of..  [#permalink]

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It was easy to eliminate I and III, but I initially thought that II was correct.

Thankfully "II only" was not in the answer choices, which made me think again. Then I realized that if the last digit was 9, then II will be violated.
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Posts: 13277
Re: For each positive integer n, p(n) is defined to be the product of..  [#permalink]

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