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Source: — Problem Solving |
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by GMATGuruNY » Tue Mar 08, 2016 11:33 am
John fran kennedy wrote:If 1/x - 1/(x+1) = 1/(x+4), then x could be?
A. 0
B. -1
C. -2
D. -3
E. -4
We can PLUG IN THE ANSWERS, which represent the value of x.
Since division by 0 is not allowed, eliminate A (which would make 1/x = 1/0), B (which would make 1/(x+1) = 1/0) and E (which would make 1/(x+4) = 1/0).

Answer choice C: x=-2
Plugging x=-2 into 1/x - 1/(x+1) = 1/(x+4), we get:
1/(-2) - 1/(-2+1) = 1/(-2+4)
-(1/2) + 1 = 1/2.
1/2 = 1/2.
Success!

The correct answer is C.
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by [email protected] » Tue Mar 08, 2016 10:36 pm
Hi John fran kennedi,

For anyone who's interested, this question can be found in the OG:

GMAT2016 - page 184, question 220
OG13/GMAT2015 - page 183, question 216

TESTing THE ANSWERS (the approach that Mitch used) is perfect for this question, so I won't rehash any of that work here. If you've looked at the explanation in the OG, then you'll notice how it focuses on the longer, technical math involved - and that approach is not always practical on Test Day (when the clock is ticking). As you continue to study, you should look for opportunities to use Tactics, pattern-matching, etc. to your advantage.

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by John fran kennedi » Wed Mar 09, 2016 1:40 am
Some of explanations from the OG book is complicated for me, that's why I do need a faster approach to solve every question and to save the time. By the way I thank you so much.
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by [email protected] » Wed Mar 09, 2016 10:02 am
Hi John fran kennedi,

You'll likely improve, speed up, etc. as you continue to study, but there are also resources that can help you to improve.

1) How long have you been studying?
2) What resources have you been using?
3) What is your goal score?
4) When are you planning to take the GMAT?

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by Brent@GMATPrepNow » Wed Mar 09, 2016 10:10 am
John fran kennedi wrote:Some of explanations from the OG book is complicated for me, that's why I do need a faster approach to solve every question and to save the time.
\

The various Official Guides are great resources for official practice questions, but they're not very comprehensive when it comes to instruction or the way the solutions are presented (often algebra-heavy). To perform well on the GMAT, you need to learn all of the core concepts AND you need to learn GMAT-specific strategies for answering questions quickly and accurately. In my opinion, the Official Guides do not provide this level of instruction.

Lucky for you, this site is packed with companies/people who can provide the instruction you need :-)

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by John fran kennedi » Wed Mar 09, 2016 10:39 am
Hi rich

I have been studying the GMAT for 2 moths.
I would like to take the GMAT test in November.
I study the GMAT from my OG book 2016 and internet.
My goal is to get the least score 760.
I know how difficult it is, but I will never give up trying to achieve that. I will always study hard
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by [email protected] » Wed Mar 09, 2016 8:52 pm
Hi John fran kennedi,

I've sent you a PM with some suggestions.

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by Matt@VeritasPrep » Thu Mar 17, 2016 9:34 pm
One other approach:

To clear the denominators in an equation, we can multiply the entire equation by all three denominators. For instance,

(1/3) + (1/2) = (1/x)

becomes

(3*2*x)(1/3) + (3*2*x)(1/2) = (3*2*x)(1/x)

becomes

2x + 3x = 6

or x = 6/5.

Our problem can use much the same approach:

1/x = 1/(x+1) + 1/(x+4)

becomes

(x+1)(x+4) = x(x+4) + x(x+1)

or

x² + 5x + 4 = x² + 4x + x² + x

or

4 = x²

So x = 2 or - 2, and the only answer choice that fits is C.
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