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BrentGMATPrepNow
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Is the positive integer N a perfect square? (1) The number 19 Nov 2015, 15:26
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Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!

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Question Stats:

86% (01:39) correct 14% (00:32) wrong based on 38 sessions

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Is the positive integer N a perfect square?

(1) The number of distinct factors of N is even.
(2) The sum of all distinct factors of N is even.
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Are you sure you correctly transcribed the question?
As it is currently worded, the answer to this question is E.
The answer WOULD be D, if we changed "distinct factors" to "POSITIVE distinct factors."

When asking questions about factors (aka divisors), the GMAT typically restricts the discussion to POSITIVE factors/divisors. If we don't specify such a restriction, then we must also consider negative factors.

From the Official Guide:
Quote:
An integer is any number in the set {. . . –3, –2, –1, 0, 1, 2, 3, . . .}.
If x and y are integers and x ≠ 0, then x is a divisor (factor) of y provided that y = xn for some integer n. In this case, y is also said to be divisible by x or to be a multiple of x.
For example, 7 is a divisor or factor of 28 since 28 = (7)(4), but 8 is not a divisor of 28 since there is no integer n such that 28 = 8n.
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So, for example, the -2 is a factor of 6 since 6 = (-2)(-3)

Now onto the question....

----------------------------------------

Target question: Is the positive integer N a perfect square?

Statement 2: The number of distinct factors of N is even
There are infinitely many values of N that satisfy this condition. Here are two:
Case a: N = 3. The distinct factors of N are {-3, -1, 1, 3}. As you can see, there is an even number of distinct factors of N. In this case N is NOT a perfect square
Case b: N = 4. The distinct factors of N are {-4, -2, -1, 1, 2, 4}. As you can see, there is an even number of distinct factors of N. In this case N IS perfect square
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: The sum of all distinct factors of N is even
There are infinitely many values of N that satisfy this condition. Here are two:
Case a: N = 3. The distinct factors of N are {-3, -1, 1, 3}, so the sum = (-3) + (-1) + 1 + 3 = 0. The sum of the distinct factors = 0, which is EVEN. In this case N is NOT a perfect square
Case b: N = 4. The distinct factors of N are {-4, -2, -1, 1, 2, 4}, so the sum = (-4) + (-2) + (-1) + 1 + 2 + 4 = 0. The sum of the distinct factors = 0, which is EVEN. In this case N IS a perfect square
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
In both cases, I showed that N COULD equal 3 or 4.
So, when we combine the statements, N COULD still equal 3 or 4.
3 is NOT a perfect square, and 4 IS a perfect square.
Since we cannot answer the target question with certainty, the combined statements are NOT SUFFICIENT

Answer = E

Cheers,
Brent

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Is the positive integer N a perfect square? (1) The number 01 Jun 2023, 22:38
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