M and N are both nonzero integers. Is M/N a prime number?

Question

(1) |M|=–M 
(2) |N|=N

I dont have the OA on this.Can someone please verify my approach .I am weak in modulus and have only recently started to look at this topic :

from 1 : since |M| has to be +ve ,and |M| = - M ,we know M is -ve. Insuff,no info on N

from 2 : |N| = N => N can be =+ve or -ve.Insuff .No info on M

1 and 2 together : M/N can be +ve or -ve (assuming prime no.s are positive) therefore E.

eschn3am · Answer

1. Tells you that M is a negative integer.
2. Tells you that N is a positive integer. (try plugging in a negative number, it doesn't work. N must be positive)

Now it gets interesting.

Taken together these statements tell us that M/N is a negative number. Since we have no info about what the integers actually are, we can't do much.

UNLESS... let's say M/N = -7 

Can you say that -7 is NOT prime because you can use (-7, 1) AND (7, -1) to get there? I don't know all the rules dealing with negative primes, but since we have no specifics this is the only thing I can come up with.

The answer is either E since we have no specifics or C because all negative numbers have at least 4 divisors. so you cannot have a negative prime.

racha24 · Answer

this is an interesting question. Although there is no unity of opinion on whether negative numbers can be prime, I think the answer is still E. But it would be interesting to know what GMAT test makers actually think about this. Any thoughts?

bkk145 · Answer

First, I am quite sure that negative number can't be prime. For this question, I get E. For M/N to be a prime, M must equal a prime and N equals 1 (1) |M| = -M if M>0, M = -M If M<0, -M = -M => M=M So M can be either negative or positive, but we still don't know N. INSUFFICIENT (2) |N|=N if N>0, N=N if N<0, N=-N Don't know M. INSUFFICIENT Together, I still can't draw any conclusion.

eschn3am · Answer

1. M  must  be negative. Since |M| must be a positive then - M must be positive as well. The only way for - M to be positive is if M is negative itself.

2. If a negative number cannot be prime than the answer would be C. M/N is 100% for certain a negative number in this problem. If negatives cannot be primes and it's asking if M/N is prime we can answer this 100% no. So if the GMAT takes the stance that there cannot be negative primes the answer would be C.

3. "for M/N to be a prime, M must equal a prime and N must equal 1" this isn't true at all. 14/2 = 7 which is a prime. 33/3 = 11 which is a prime. There are tons of ways to divide two numbers and come up with a prime. The only way you can get a prime number through  multiplication  is if one number is prime and the other is one (this is the very definition of a prime number).

bkk145 · Answer

Yea, I think I need to. When I do the math problems too fast or at work or both, I make really stupid mistakes. Then I look back and I am like, what? I did this? Happened to me twice today.

Regarding negative prime, my understanding is that any negative number (except -1) has at least 3 factors. Say -3 = 1*3*-1, which violates the definition of prime number. For -1, we can have multiple factors: -1 = -1*-1*-1*1 and so on; hence, not a prime either.

Fistail · Answer

but in gmat, -ve is not considered prime. 
can any one confirm?
thanx.

ywilfred · Answer

St1:
Modula of any number is always positive, so M must be negative. We have no idea about N, so st1 is insufficient.

St2:
N must be a positive integer, but nothing M is known, so st2 is insufficient.

Using both,
M/N will always be negative, so M/N is not a prime number since the smallest prime number is a positive number. Sufficient.

Ans C

ragz · Answer

If |M| = -M doesn't it mean that we still are not sure whether -M is negative or positive?
if M=-2 then -M=2
if M=2 then -M=-2

Correct me if I'm wrong

sumande · Answer

I had never heard about negative primes. And in fact they do not exist by definition:
 A prime number (or prime integer, often simply called a "prime" for short) is a positive integer p>1 that has no positive integer divisors other than 1 and p itself.  

 https://mathworld.wolfram.com/PrimeNumber.html

eschn3am · Answer

|M| by definition must be a positive number since all absolute values are positive. So the left side of the equation is positive here. That means the right side of the equation must be positive as well. Plug in numbers into -M and see what you get. -(10) = -10 but -(-10) = 10

Example 1: (negative number)

|-10| = - (-10)
10 = 10
CORRECT

Example 2: (positive number)

|10| = -(10)
10 = -10
INCORRECT

M has no choice but to be negative here.

FN · Answer

I am with C on this...

there is no such thing as a negative prime number...

gmathacker1 · Answer

yes negative numbers cant be prime numbers, Manhatten GMAT books confirm that

shiblee · Answer

I will go with C .

1) it just tells us "M " is a negative or 0 . because |M| is a positive so -M must be positive . in order to do so , M must be negative .

here we are told that M is not zero so M is negative .

we do not know anything about N so it is not sufficient

2) it tells us N is postive . but no info about M . hence not sufficient

1 + 2 together

M/N is a negative number . A negative number can not be a prime so M/N is not a prime . hence sufficient .

correct answer = C

Posted from my mobile device

Bunuel · Answer

Just to close discussion about negative primes. There is NO such thing. Primes are positive integers by definition. Check more tips on Number Properties here: tips-and-hints-for-specific-quant-topics-with-examples-172096.html#p1371030

bumpbot · Answer

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M and N are both nonzero integers. Is M/N a prime number?

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