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# The product of two negative integers, a and b, is a prime nu

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Math Expert
Joined: 02 Sep 2009
Posts: 53066
The product of two negative integers, a and b, is a prime nu  [#permalink]

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03 Jun 2014, 04:55
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20
00:00

Difficulty:

95% (hard)

Question Stats:

44% (02:13) correct 56% (02:12) wrong based on 628 sessions

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The product of two negative integers, $$a$$ and $$b$$, is a prime number $$p$$. If $$p$$ is the number of factors of $$n$$, where $$n$$ is NOT a perfect square, what is the value of the median of the four integers $$a$$, $$b$$, $$p$$, and $$n$$?

A. 0
B. 1/2
C. 1
D. 3/2
E. 2

Kudos for a correct solution.

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Math Expert
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Posts: 53066
Re: The product of two negative integers, a and b, is a prime nu  [#permalink]

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03 Jun 2014, 04:55
8
10
SOLUTION

The product of two negative integers, $$a$$ and $$b$$, is a prime number $$p$$. If $$p$$ is the number of factors of $$n$$, where $$n$$ is NOT a perfect square, what is the value of the median of the four integers $$a$$, $$b$$, $$p$$, and $$n$$?

A. 0
B. 1/2
C. 1
D. 3/2
E. 2

This is a hard questions which tests several number theory concepts.

Start from n: we are told that $$n$$ is NOT a perfect square. The number of factors of perfect square is odd and all other positive integers have even number of factors. Hence, since $$p$$ is the number of factors of $$n$$, then $$p$$ must be even. We also know that $$p$$ is a prime number and since the only even prime is 2, then $$p=2$$. Notice here that from this it follows that $$n$$ must also be a prime, because only primes have 2 factors: 1 and itself.

Next, $$ab=p=2$$ implies that $$a=-1$$ and $$b=-2$$ or vise-versa.

So, the set is {-2, -1, 2, some prime}, which means that the median is (-1 + 2)/2 = 1/2.

Theory on Number Properties: math-number-theory-88376.html
Tips and hints about Number Properties

DS Number Properties Problems to practice: search.php?search_id=tag&tag_id=38
PS Number Properties Problems to practice: search.php?search_id=tag&tag_id=59

Please share your number properties tips HERE and get kudos point. Thank you.
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Re: The product of two negative integers, a and b, is a prime nu  [#permalink]

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03 Jun 2014, 07:05
2
Bunuel wrote:

The product of two negative integers, $$a$$ and $$b$$, is a prime number $$p$$. If $$p$$ is the number of factors of $$n$$, where $$n$$ is NOT a perfect square, what is the value of the median of the four integers $$a$$, $$b$$, $$p$$, and $$n$$?

A. 0
B. 1/2
C. 1
D. 3/2
E. 2

Kudos for a correct solution.

all prime numbers are divisible by 1 and itself. now since product of a and b is a prime number P. therefore one of a and b is -1 and other is -P.

Also, all the perfect square have odd no. of factors. for e.g. a^2 will have 3 factors a,a^2 and 1. Since, n is not a perfect square therefore its no. of factors must be even and the only prime no. which is even is 2. therefore p=2,

now the median of numbers -2,-1,2 and n will be (-1+2)/2= 1/2 hence B
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Re: The product of two negative integers, a and b, is a prime nu  [#permalink]

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04 Jun 2014, 09:54
1
Bunuel wrote:

The product of two negative integers, $$a$$ and $$b$$, is a prime number $$p$$. If $$p$$ is the number of factors of $$n$$, where $$n$$ is NOT a perfect square, what is the value of the median of the four integers $$a$$, $$b$$, $$p$$, and $$n$$?

A. 0
B. 1/2
C. 1
D. 3/2
E. 2

Kudos for a correct solution.

We know that the only way two integers will give us a prime is 1 times a prime number. So A or B is -1 and know both numbers are negative.

Since N is not a perfect square it should have 2 factors. Since P=the number of factors, 2, and A or B is -1, then A or B must also be -2.

A*B=2
A=-1
B=-2
P=2
P=N=2

Set of Numbers {-2,-1,2,2}

Median = ((-1+2)/2) =1/2

Math Expert
Joined: 02 Sep 2009
Posts: 53066
Re: The product of two negative integers, a and b, is a prime nu  [#permalink]

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04 Jun 2014, 11:09
GeorgeA023 wrote:
Bunuel wrote:

The product of two negative integers, $$a$$ and $$b$$, is a prime number $$p$$. If $$p$$ is the number of factors of $$n$$, where $$n$$ is NOT a perfect square, what is the value of the median of the four integers $$a$$, $$b$$, $$p$$, and $$n$$?

A. 0
B. 1/2
C. 1
D. 3/2
E. 2

Kudos for a correct solution.

We know that the only way two integers will give us a prime is 1 times a prime number. So A or B is -1 and know both numbers are negative.

Since N is not a perfect square it should have 2 factors. Since P=the number of factors, 2, and A or B is -1, then A or B must also be -2.

A*B=2
A=-1
B=-2
P=2
P=N=2

Set of Numbers {-2,-1,2,2}

Median = ((-1+2)/2) =1/2

Notice that it's not necessary n to be 2, it could be any other prime as well.
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Math Expert
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Posts: 53066
Re: The product of two negative integers, a and b, is a prime nu  [#permalink]

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04 Jun 2014, 11:09
1
1
SOLUTION

The product of two negative integers, $$a$$ and $$b$$, is a prime number $$p$$. If $$p$$ is the number of factors of $$n$$, where $$n$$ is NOT a perfect square, what is the value of the median of the four integers $$a$$, $$b$$, $$p$$, and $$n$$?

A. 0
B. 1/2
C. 1
D. 3/2
E. 2

This is a hard questions which tests several number theory concepts.

Start from n: we are told that $$n$$ is NOT a perfect square. The number of factors of perfect square is odd and all other positive integers have even number of factors. Hence, since $$p$$ is the number of factors of $$n$$, then $$p$$ must be even. We also know that $$p$$ is a prime number and since the only even prime is 2, then $$p=2$$. Notice here that from this it follows that $$n$$ must also be a prime, because only primes have 2 factors: 1 and itself.

Next, $$ab=p=2$$ implies that $$a=-1$$ and $$b=-2$$ or vise-versa.

So, the set is {-2, -1, 2, some prime}, whcih means that the median is (-1 + 2)/2 = 1/2.

Theory on Number Properties: math-number-theory-88376.html
Tips and hints about Number Properties

DS Number Properties Problems to practice: search.php?search_id=tag&tag_id=38
PS Number Properties Problems to practice: search.php?search_id=tag&tag_id=59
_________________
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Posts: 39
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Re: The product of two negative integers, a and b, is a prime nu  [#permalink]

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04 Jun 2014, 11:13
Bunuel wrote:
GeorgeA023 wrote:
Bunuel wrote:

The product of two negative integers, $$a$$ and $$b$$, is a prime number $$p$$. If $$p$$ is the number of factors of $$n$$, where $$n$$ is NOT a perfect square, what is the value of the median of the four integers $$a$$, $$b$$, $$p$$, and $$n$$?

A. 0
B. 1/2
C. 1
D. 3/2
E. 2

Kudos for a correct solution.

We know that the only way two integers will give us a prime is 1 times a prime number. So A or B is -1 and know both numbers are negative.

Since N is not a perfect square it should have 2 factors. Since P=the number of factors, 2, and A or B is -1, then A or B must also be -2.

A*B=2
A=-1
B=-2
P=2
P=N=2

Set of Numbers {-2,-1,2,2}

Median = ((-1+2)/2) =1/2

Notice that it's not necessary n to be 2, it could be any other prime as well.

Bunuel,

Yeah I see that now. When I read it I glanced over the second of. I saw P is the number of factors N and not P is the number of factors of N. So I read it as P=N and not the way it was written.
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Re: The product of two negative integers, a and b, is a prime nu  [#permalink]

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05 Jun 2014, 22:20
Assumed numbers:

a, b, p, n as

-2, -1, 2, 3

-2 * -1 = 2 (Prime Number)

3 has 2 factors (1 & 3);

It is not a square

Median $$= \frac{2-1}{2} = \frac{1}{2}$$

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Re: The product of two negative integers, a and b, is a prime nu  [#permalink]

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01 Jan 2018, 06:18
given: 1. a * b = prime number
2. a and b -ve numbers

so one of them has to be -1
say a = -1

probable ascending order: { b , a, p , n}
since p is number of factors of n, where n is not a perfect square. p cannot be odd (as only perfect square has odd number of factors)

so p is even, so median of {b,a,p,n} = (even - 1)/2 = odd / 2 = fraction.

Only option B and D left.

if median were 3/2, then p = 4 => not a prime => so D is out

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The product of two negative integers, a and b, is a prime nu  [#permalink]

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25 Feb 2018, 20:04
Excellent Excellence question!!!

a*b is prime.
Since, prime number(P) only has 2 factors 1 and P and any factor of P must contain of product that arrives at P.
The only way the product is prime if one of the number is prime (the same number) and other is 1. In this case both negative.
so, a = -1 and B = -P (or vice versa).

Now, N is not a perfect square so the number of factor has to be even. The only way that p is both even and prime is if it is 2.

so, a =-1,B=-2 and p =2.

I got kind of stuck here and could not determine n. But was pretty sure it is -2 or 2.

if it is 2 then the set is -2,-1,2,2 . median is 1/2.. MY answer
if it is -2, then set is -2,-2,-1,2 median is -3/2. not one of the option..
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The product of two negative integers, a and b, is a prime nu  [#permalink]

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25 Feb 2018, 23:12
A really tough question, took me a good 7 min to solve.

The product of two negative integers, a and b, is a prime number p.

If p is a prime number, a is -1 and b is -p, or vice versa.

If p is the number of factors of n, where n is NOT a perfect square,

A non-perfect square integer will always have even number of factors. And the only even prime number is 2. Hence a & b is -1 & -2 or vice versa.

what is the value of the median of the four integers a, b, p, and n

We know the following values are present -2, -1, 2, n.
Since 2<n and the question is asking about median, we do not need to know the exact value of n.
The answer is $$(-1+2)/2 = 1/2$$

Ans: B
The product of two negative integers, a and b, is a prime nu   [#permalink] 25 Feb 2018, 23:12
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