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Director  V
Joined: 18 Feb 2019
Posts: 603
Location: India
GMAT 1: 460 Q42 V13 GPA: 3.6
Is the positive integer n a multiple of 40?  [#permalink]

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5 00:00

Difficulty:   85% (hard)

Question Stats: 43% (02:16) correct 57% (02:29) wrong based on 42 sessions

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Is the positive integer n a multiple of 40?

I. 20 is a factor of n^2
II. n^3/128 is an integer.
Manager  G
Joined: 16 Oct 2011
Posts: 109
GMAT 1: 570 Q39 V41 GMAT 2: 640 Q38 V31 GMAT 3: 650 Q42 V38 GMAT 4: 650 Q44 V36 GMAT 5: 570 Q31 V38 GPA: 3.75
Re: Is the positive integer n a multiple of 40?  [#permalink]

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kiran120680 wrote:
Is the positive integer n a multiple of 40?

I. 20 is a factor of n^2
II. n^3/128 is an integer.

We are asked if positive integer n is a multiple of 40

(1) 20 is a factor of n^2 20 = 2*2*5 We cannot take the square root of 5 and still have an integer, therefore min n^2 = 2*2*5*5 and min n =2*5 = 10. Therefore n = 10,20,30,40.... If n = 10 The answer is NO. if n = 40, the answer is YES NS

(2) n^3 is divisible by 128. 128 factors to 2^7. Now, in order for n to be an integer, (n^3)^1/3 must be an integer. Notice that (2^7)^(1/3) is not an integer. We need our exponent on base 2 to be divisible by 3, therefore min n^3 = 2^9 and min n=2^3 =8. If n = 8 the answer is no, but if n = 5*8 = 40 the answer is yes. NS

(1) and (2) n is a multiple of 10 and n is a multiple of 8. Therefore n is a multiple of the least common multiple of 10 and 8. 10 = 2*5 and 8 =2*2*2. Therefore n is a multiple of 2*2*2*5 = 8*5 = 40. Sufficient.

##### General Discussion
Intern  B
Joined: 21 Apr 2014
Posts: 32
Re: Is the positive integer n a multiple of 40?  [#permalink]

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I think of these "is X a multiple of Y?" questions as: "does X contain ALL of Y's factors?"

40 = 2 x 2 x 2 x 5, or three 2's and one 5.

This is a Yes or No question.
If Yes, then "n" will have three 2's and one 5 as factors.
If No, then "n" will be account for all factors.

We don't care whether the answer is actually "yes" or "no" (no horse in this race! ), but we just need it to be 100% certain.

1) 20 is a factor of n^2

Okay, so n^2 = n x n, and they are saying that n x n will have all the factors of 20. 20 = 2 x 2 x 5

So basically 2 x 2 x 5 would evenly divide into n x n. But hang on a minute, we have two n's in that hypothetical numerator, and the factors don't evenly split up! This tells us that "n" must have AT LEAST one 2 and one 5 as factors. So, it's possible the answer is YES, if n also has two more 2's, but what if n = 10? Then we'd get a NO answer. This is insufficient. Cross off answer choices A and D.

Let's apply the same logic to Statement 2:

2) n^3/128 is an integer.

128 = 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2, since we're splitting these eight 2's amongst three n's, it must be that we're missing a 2, and that each "n" has AT LEAST three 2's in it. What we don't know anything about is whether or not it has a 5. If it does, the answer is YES. If it doesn't, the answer is NO. This is insufficient; cross off answer choice B.

Combined:

"n" must have one 5 as per Statement 1, and "n" must have three 2's as per Statement 2. Therefore the answer will always be YES, that n is a multiple of 40. (Also, remember that every number is factor and a multiple of itself. So let's say n = 40. 40 is a multiple of 40, so that would give us a YES answer.)

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Reading Comprehension is my jam!  Re: Is the positive integer n a multiple of 40?   [#permalink] 01 Mar 2019, 23:45
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