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I made this mistake while solving a ques.Throw some light?

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I made this mistake while solving a ques.Throw some light? [#permalink]  19 Jul 2009, 19:01
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Guys I was solving a question and I made this silly mistake.I corrected myself but was hoping somebody throw some light on where my concept was wrong.

Suppose 2/3 out of N employees dring regular coffee.Out of these 2/5 add cream.How many of the employees who drink regular coffee do not add cream?
Ans: 2/3n - 2/5n = 4n/15.(ans.)

What I did was:If 2/5 add cream,then 3/5 do not add cream.So 3/5 x 2/3n = 2/5n
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Re: I made this mistake while solving a ques.Throw some light? [#permalink]  19 Jul 2009, 21:23
tejal777 wrote:
Guys I was solving a question and I made this silly mistake.I corrected myself but was hoping somebody throw some light on where my concept was wrong.

Suppose 2/3 out of N employees dring regular coffee.Out of these 2/5 add cream.How many of the employees who drink regular coffee do not add cream?
Ans: 2/3n - 2/5n = 4n/15.(ans.)

What I did was:If 2/5 add cream,then 3/5 do not add cream.So 3/5 x 2/3n = 2/5n

Catch some light. Sorry, can't help !

When you say 2/3 employees of total. It is $$2N/3$$.

When you calculated 3/5 = do not add cream. It means $$3N/5$$ do not add cream. It's $$3/5$$ of total employees & not $$3/5$$ of 'the employees who drink regular coffee'.

Let's say $$Y = 2N/3$$ = employees who drink regular coffee.

You are supposed to multiply $$3/5$$ to 'N' not to Y.

P.S. You can draw a venn diagram, if you are still confused. I am too lazy to draw one on Monday morning. :yawn:
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Re: I made this mistake while solving a ques.Throw some light? [#permalink]  19 Jul 2009, 22:09
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"Suppose 2/3 out of N employees dring regular coffee." => 2/3 drink regular coffee
"Out of these 2/5 add cream." of those who drink regular coffee (2/3), 2/5 add cream and 3/5 do not add cream. Hense of all employees 2/3 * 2/5 drink regular coffee and add cream, 2/3 * 3/5 drink regular coffea and dont add cream

"How many of the employees who drink regular coffee do not add cream?"
Answer should be 3/5. No need for multiplcation because we care about only employees who drink regular coffee.
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Re: I made this mistake while solving a ques.Throw some light? [#permalink]  19 Jul 2009, 22:16
Expert's post
Welcome to GMAT Club PussInBoots!

I could not pass those "puppy" eyes. Ask anything you want
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Manager
Joined: 19 Jul 2009
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Schools: McIntire, Fuqua, Villanova, Vandy, WashingtonU
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Re: I made this mistake while solving a ques.Throw some light? [#permalink]  19 Jul 2009, 22:21
bb wrote:
Welcome to GMAT Club PussInBoots!

I could not pass those "puppy" eyes. Ask anything you want

Can I have that V42 of yours?
_________________

12/23 GMATPrep #1: 730 q50 v38
12/26 MGMAT CAT #1: 730 q50 v40
01/01 MGMAT CAT #2: 710 q51 v36
01/12 Real GMAT #1: 690... WTF?!?!?!?!?!?!
...
02/28 Real GMAT #2: ...

Founder
Affiliations: AS - Gold, UA - Silver, HH-Diamond
Joined: 04 Dec 2002
Posts: 12821
Location: United States (WA)
GMAT 1: 750 Q49 V42
GPA: 3.5
WE: Information Technology (Hospitality and Tourism)
Followers: 2725

Kudos [?]: 13517 [0], given: 3865

Re: I made this mistake while solving a ques.Throw some light? [#permalink]  19 Jul 2009, 22:24
Expert's post
PussInBoots wrote:
bb wrote:
Welcome to GMAT Club PussInBoots!

I could not pass those "puppy" eyes. Ask anything you want

Can I have that V42 of yours?

_________________

Founder of GMAT Club

Just starting out with GMAT? Start here... | Want to know your GMAT Score? Try GMAT Score Estimator
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Re: I made this mistake while solving a ques.Throw some light? [#permalink]  20 Jul 2009, 02:19
Quote:
Suppose 2/3 out of N employees dring regular coffee.Out of these 2/5 add cream.How many of the employees who drink regular coffee do not add cream?

Consider a number which is multiple of LCM of 3,5 ( Denominators of fraction in question) say 30.

2/3 of N i.e. 2/3 of 30 go for regular coffee. : 20 .
Out of these(20) 2/5 add cream: 8.

How many who drink do not add cream/ Total coffee drinkers: 20-8: 12/20 = 3/5*N
Re: I made this mistake while solving a ques.Throw some light?   [#permalink] 20 Jul 2009, 02:19
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