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Is the positive Integer x a perfect square? 1) Sum of all factors of  [#permalink]

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Difficulty:   75% (hard)

Question Stats: 47% (01:27) correct 53% (01:33) wrong based on 116 sessions

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Is the positive Integer x a perfect square?

1) Sum of all factors of x is odd
2) Number of all factors of x is odd

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Re: Is the positive Integer x a perfect square? 1) Sum of all factors of  [#permalink]

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B.

A doesnt fit. For a number to be perfect square only the number of factors has to be odd not the sum. Insufficient
B exactly states the rule. sufficient
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Re: Is the positive Integer x a perfect square? 1) Sum of all factors of  [#permalink]

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1
GMATinsight wrote:
Is the positive Integer x a perfect square?

1) Sum of all factors of x is odd
2) Number of all factors of x is odd

Statement 2:

It is a property worth remembering:

All perfect square has odd number of factors and vice versa.

4 has positive factors: 1,2 ,4..Sum = 7

9 has positive factors: 1,3 ,9..Sum = 13

Sufficient

Statement 1:

Use one example from above

4 has positive factors: 1,2 ,4..Sum = 7..Answer is Yes

2 has positive factors: 1,2..Sum = 3 .....Answer is No

Insufficient

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GMAT 1: 750 Q50 V42 Is the positive Integer x a perfect square? 1) Sum of all factors of  [#permalink]

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Statement 1: a perfect square will have odd number of odd factors and even number of even factors. Therefore odd*odd + even*even = odd , however, vice versa is not true. It isn't necessary that if sum of factors is odd then it is a perfect square. Say number 6. 6 = 3*2, sum of factors = 5 but is 6 a perfect square. NO.

Lets say the number is 8. Sum of factors (1+2+4+8)=15 and its not a perfect square.

Insufficient

Statement 2: the perfect square will always have odd no of factors. How?

a^2*b^2 = total no of factors = 3*3 = 9 ( odd)
a^4*b^4 = total no of factors = 5*5= 25 ( odd)
Square of Prime number has 3 factors a,1,a^2
Sufficient

Hence Ans = B

Posted from my mobile device

Originally posted by Mudit27021988 on 03 Mar 2018, 11:55.
Last edited by Mudit27021988 on 03 Sep 2019, 09:31, edited 1 time in total.
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Re: Is the positive Integer x a perfect square? 1) Sum of all factors of  [#permalink]

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1
[quote="Mudit27021988"]Statement 1: a perfect square will have odd number of odd factors and even number of even factors. Therefore odd*odd + even*even = odd , however, vice versa is not true. It isn't necessary that if sum of factors is odd then it is a perfect square. Say number 6. 6 = 3*2, sum of factors = 5 but is 6 a perfect square. NO.
Insufficient

Statement 2: the perfect square will always have odd no of factors. How?

a^2*b^2 = total no of factors = 3*3 = 9 ( odd)
a^4*b^4 = total no of factors = 5*5= 25 ( odd)
Square of Prime number has 3 factors a,1,a^2
Sufficient

Hence Ans = B

Posted from my mobile device[/quote

shouldnt the sum of factors of 6 be....2+3+6+1=12

i think the only insuffiicient condition for statement 1 is the number 2
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GMAT 1: 750 Q50 V42 Re: Is the positive Integer x a perfect square? 1) Sum of all factors of  [#permalink]

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1
avikroy wrote:
Mudit27021988 wrote:
Statement 1: a perfect square will have odd number of odd factors and even number of even factors. Therefore odd*odd + even*even = odd , however, vice versa is not true. It isn't necessary that if sum of factors is odd then it is a perfect square. Say number 6. 6 = 3*2, sum of factors = 5 but is 6 a perfect square. NO.
Insufficient

Statement 2: the perfect square will always have odd no of factors. How?

a^2*b^2 = total no of factors = 3*3 = 9 ( odd)
a^4*b^4 = total no of factors = 5*5= 25 ( odd)
Square of Prime number has 3 factors a,1,a^2
Sufficient

Hence Ans = B

Posted from my mobile device[/quote

shouldnt the sum of factors of 6 be....2+3+6+1=12

i think the only insuffiicient condition for statement 1 is the number 2

You are right.
Thanks for highlighting. Updated the post.

Regards
Mudit
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Re: Is the positive Integer x a perfect square? 1) Sum of all factors of  [#permalink]

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Because a perfect square always has a pair of prime factors, when prime factorized, a perfect square will always yield even powers for its prime factors. What this essentially means is that, perfect squares are the only positive integers which have an odd number of factors.

Additionally, a perfect square will always have odd number of odd factors and even number of even factors. If you come to think about it, this is the only way in which the total number of factors can be odd, because,

Total factors = Odd factors + Even factors; this implies Odd = Odd + Even (since the total number of factors is odd)

For example, 16 is a perfect square. The factors of 16 are 1, 4 and 16. It has ONE odd factor and TWO even factors.

36 is a perfect square. The factors of 36 are 1, 2,3, 4, 6, 9, 12, 18 and 36. There are 3 odd factors viz 1,3 and 9;there are 6 even factors viz 2,4,6,12,18 and 36. The total number of factors is 9.

Therefore, whenever you have a question asking you whether a given number is a perfect square, remember that, one way of identifying whether it is or not, is by looking at the number of factors it has.

With this in mind, let us look at the statements.

From statement I, we know that the sum of all factors of the number is odd.

If the number is 2, the sum of all its factors = 1 + 2 = 3. 2 is not a perfect square.

If the number is 4, the sum of all its factors = 1 + 2 + 4 = 7. 4 IS a perfect square.

Statement I is clearly insufficient. Answer options A and D can be ruled out, possible answer options are B, C or E.

From statement II, we know that the number of all factors of the number is odd. This is possible only when the number is a perfect square. So we can answer the question with a definite YES.

Statement II alone is sufficient. The correct answer has to be B.

Hope this helps!
_________________ Re: Is the positive Integer x a perfect square? 1) Sum of all factors of   [#permalink] 03 Sep 2019, 23:22
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