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Equations

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Intern
Joined: 27 Mar 2019
Posts: 2

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11 Aug 2019, 11:30
3X+5Y+2Z=20
6X+4Z=10

What is y?

I know that you divide the bottom equation by 2 and get 3X+2Z=5 and then SUBTRACT the two equations for the answer to be Y=3.

My question is why subtract and not add.

How do you know when to do either approach? If you added you would get Y=5.

Besides plugging in to find out (because it is a long process) how would you know that you safely either added or subtracted in that earlier step?
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Status: Manager
Joined: 27 Oct 2018
Posts: 803
Location: Egypt
GPA: 3.67
WE: Pharmaceuticals (Health Care)

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11 Aug 2019, 18:06
hannahf11 wrote:
3X+5Y+2Z=20
6X+4Z=10

What is y?

I know that you divide the bottom equation by 2 and get 3X+2Z=5 and then SUBTRACT the two equations for the answer to be Y=3.

My question is why subtract and not add.

How do you know when to do either approach? If you added you would get Y=5.

Besides plugging in to find out (because it is a long process) how would you know that you safely either added or subtracted in that earlier step?

Hi hannahf11
Why not adding? Because it won't help to eliminate some of the unknown variables
if you add you would get Y=5? Impossible. you must be missing something. ( revise your approach, or show me your steps to see how you got 5)
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Joined: 15 Jan 2018
Posts: 909
Concentration: General Management, Finance
GMAT 1: 720 Q50 V37

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11 Aug 2019, 22:34
hannahf11 - Welcome to GMAT Club and thank you for the query. Our forum member, Mahmoudfawzy83, has beautifully answered your query. That said, I would advice you to browse through the various forum to find the various areas. All quant related questions must be posted in the Quantitative forum.
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Intern
Joined: 27 Mar 2019
Posts: 2

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12 Aug 2019, 03:36
Thank you for your answer you're absolutely right I wont be able to cancel if I add. I see it now!! Smh lol and next time I'll post in the quant forum.

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Re: Equations   [#permalink] 12 Aug 2019, 03:36