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Math Expert V
Joined: 02 Sep 2009
Posts: 55271
If m and n are integers, what is the value of m + n ?  [#permalink]

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Difficulty:   15% (low)

Question Stats: 77% (01:11) correct 23% (01:31) wrong based on 111 sessions

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If m and n are integers, what is the value of m + n ?

(1) (x + m)(x + n) = x^2 + 5x + mn and x ≠ 0.
(2) mn = 4

DS53602.01
OG2020 NEW QUESTION

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Re: If m and n are integers, what is the value of m + n ?  [#permalink]

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1
From S1:

$$(x + m)(x + n) = x^2 + 5x + mn$$

$$x^2+x(m+n)+mn = x^2+5x+mn$$

5x = x(m+n)

x(5-(m+n)) = 0

As x ≠ 0, m+n = 5

Sufficient.

From S2:

Clearly Insufficient.

A is the answer.
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Math Expert V
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Re: If m and n are integers, what is the value of m + n ?  [#permalink]

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1
If m and n are integers, what is the value of m + n ?

(1) (x + m)(x + n) = x^2 + 5x + mn and x ≠ 0.
(x + m)(x + n) is a quadratic equation that has m and n as roots..
$$(x + m)(x + n)=x^2+(m+n)x+mn$$..
Therefore, $$x^2+(m+n)x+mn=x^2 + 5x + mn$$
Thus m+n is the sum of roots and is equal to 5

(2) mn = 4
Mainly for C trap.
Insuff

A
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Re: If m and n are integers, what is the value of m + n ?  [#permalink]

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Bunuel wrote:
If m and n are integers, what is the value of m + n ?

(1) (x + m)(x + n) = x^2 + 5x + mn and x ≠ 0.
(2) mn = 4

DS53602.01
OG2020 NEW QUESTION

We know that $$m$$ and $$n$$ are integers. The original question: $$m+n=?$$

1) We know that $$(x+m)(x+n)=x^2+5x+mn$$ and $$x\neq 0$$.

$$x^2+mx+nx+mn=x^2+5x+mn$$

$$x(m+n)=5x$$

Since $$x\neq 0$$, we can divide both sides of the above equation by $$x$$.

$$m+n=5$$. Thus, the answer to the original question is a unique value. $$\implies$$ Sufficient

2) We know that $$mn=4$$ and can test possible cases. If $$m=2$$ and $$n=2$$, then $$m+n=4$$. However, if $$m=4$$ and $$n=1$$, then $$m+n=5$$. Thus, we can't get a unique value to answer the original question. $$\implies$$ Insufficient

Answer: A
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Re: If m and n are integers, what is the value of m + n ?  [#permalink]

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Bunuel wrote:
If m and n are integers, what is the value of m + n ?

(1) (x + m)(x + n) = x^2 + 5x + mn and x ≠ 0.
(2) mn = 4

DS53602.01
OG2020 NEW QUESTION

#1
solve for
(x + m)(x + n) = x^2 + 5x + mn and x ≠ 0.
we get m+n= 5
sufficeint
#2
mn=4
not sufficeint
IMO A
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Re: If m and n are integers, what is the value of m + n ?  [#permalink]

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Top Contributor
Bunuel wrote:
If m and n are integers, what is the value of m + n ?

(1) (x + m)(x + n) = x² + 5x + mn and x ≠ 0.
(2) mn = 4

Given: m and n are integers

Target question: What is the value of m + n ?

Statement 1: (x + m)(x + n) = x² + 5x + mn and x ≠ 0.
Use FOIL to expand the left side: x² + nx + mx + mn = x² + 5x + mn
Factor the two middle terms: x² + x(n + m) + mn = x² + 5x + mn
At this point, we should see that m+n = 5
Since we can answer the target question with certainty, statement 1 is SUFFICIENT

If you're not convinced, we can take a few more steps.
Take: x² + x(n + m) + mn = x² + 5x + mn
Subtract x² from both sides: x(n + m) + mn = 5x + mn
Subtract mn from both sides: x(n + m) = 5x
Divide both sides by x to get: n + m = 5
Since we can answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: mn = 4
There are many values of m and n that satisfy statement 2. Here are two:
Case a: m = 4 and n = 1. In this case, the answer to the target question is m + n = 4 + 1 = 5
Case b: m = 2 and n = 2. In this case, the answer to the target question is m + n = 2 + 2 = 4
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Answer: A

Cheers,
Brent
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Re: If m and n are integers, what is the value of m + n ?  [#permalink]

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Bunuel wrote:
If m and n are integers, what is the value of m + n ?

(1) (x + m)(x + n) = x^2 + 5x + mn and x ≠ 0.
(2) mn = 4

DS53602.01
OG2020 NEW QUESTION

Statement One Alone:

(x + m)(x + n) = x^2 + 5x + mn and x ≠ 0.

Foiling we have:

x^2 + mx + nx + mn = x^2 + 5x + mn

mx + nx = 5x

x(m + n) = 5x

m + n = 5

Statement one alone is sufficient to answer the question.

Statement Two Alone:

mn = 4

Only knowing the product and m and n does not give us enough information to determine the sum of n and m. For example, if m = 1 and n = 4, then m + n = 5. However, if m = n = 2, then m + n = 4. Statement two alone is not sufficient to answer the question.

Answer: A
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GMAT 1: 800 Q51 V49 GRE 1: Q170 V170 Re: If m and n are integers, what is the value of m + n ?  [#permalink]

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Hi All,

We're told that M and N are INTEGERS. We're asked for the value of M+N. This question can be solved with a mix of Algebra and TESTing VALUES.

(1) (X + M)(X + N) = X^2 + 5X + (M)(N) and X ≠ 0.

The equation in Fact 1 should get us thinking about a Quadratic and how we can F.O.I.L. the parentheses and get the given result. Notice that the 'middle' term is "+5X"; this provides a huge clue about how M and N relate to one another. Since the SUM of M and N must be +5, you can stop right here (since the question asks for that exact value). If you didn't immediately recognize that though, you can run a few TESTs and see what happens...

IF....
M=1 and N=4, then (X+1)(X+4) = X^2 +5X + 4 and the answer to the question is 5.
M=2 and N=3, then (X+2)(X+3) = X^2 +5X + 6 and the answer to the question is 5.
M=6 and N= -1, then (X+6)(X-1) = X^2 +5X - 6 and the answer to the question is 5.
Etc.
The answer to the question is ALWAYS 5.
Fact 1 is SUFFICIENT

(2) (M)(N) = 4

With the equation in Fact 2, we can come up with a couple of simple examples...
IF....
M=1 and N=4, then the answer to the question is 5.
M=2 and N=2, then the answer to the question is 4.
Fact 2 is INSUFFICIENT

Final Answer:

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www.empowergmat.com/ Re: If m and n are integers, what is the value of m + n ?   [#permalink] 20 May 2019, 13:28
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