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Arnold and Danny are two twin brothers that are...

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by swerve » Sat Jan 20, 2018 7:24 am
Arnold and Danny are two twin brothers that are celebrating their birthday. The product of their ages today is smaller by 9 from the product of their ages a year from today. What is their age today?

A. 2
B. 4
C. 5
D. 7
E. 9

The OA is B.

Please, can any expert explain this PS question for me? I tried to solve it but I can't get the correct answer. i can't define the correct equation to solve it. I need your help. Thanks.
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by EconomistGMATTutor » Sat Jan 20, 2018 1:07 pm
Arnold and Danny are two twin brothers that are celebrating their birthday. The product of their ages today is smaller by 9 from the product of their ages a year from today. What is their age today?

A. 2
B. 4
C. 5
D. 7
E. 9

The OA is B.

Please, can any expert explain this PS question for me? I tried to solve it but I can't get the correct answer. i can't define the correct equation to solve it. I need your help. Thanks.
Hi swerve,
Let's take a look at your question.

Let age of twin brothers be x today then after a year their age will be x + 1.
The product of their ages today is smaller by 9 from the product of their ages a year from today. It can be written as:
$$\left(x+1\right)^2-9=x^2$$
$$x^2+2x+1-9=x^2$$
$$2x+1-9=0$$
$$2x-8=0$$
$$2x=8$$
$$x=\frac{8}{2}=4$$

Therefore, Option B is correct.

Hope it helps.
I am available if you'd like any follow up.
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by [email protected] » Mon Jan 22, 2018 2:35 pm
Hi swerve,

We're told that Arnold and Danny are TWIN brothers who are celebrating their birthdays and the product of their ages TODAY is 9 less than the product of their ages a year from today. We're asked for their ages TODAY. This question can be solved by TESTing the ANSWERS.

Since Arnold and Danny are TWINS, they have the SAME age. Let's start with Answer B...

Answer B: 4 years old
Product of the ages TODAY = (4)(4) = 16
Product of their ages in ONE YEAR = (5)(5) = 25
The difference in products is 25 - 16 = 9
This is an exact match for what we were told, so this MUST be the answer.

Final Answer: B

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by Jeff@TargetTestPrep » Mon Jan 29, 2018 9:51 am
swerve wrote:Arnold and Danny are two twin brothers that are celebrating their birthday. The product of their ages today is smaller by 9 from the product of their ages a year from today. What is their age today?

A. 2
B. 4
C. 5
D. 7
E. 9
If we let Arnold's and Danny’s age today = n, then their ages one year from today will be (n + 1). Thus, we have:

n^2 + 9 = (n + 1)(n + 1)

n^2 + 9 = n^2 + 2n + 1

8 = 2n

4 = n

Answer: B

Jeffrey Miller
Head of GMAT Instruction
[email protected]

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