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

This topic has expert replies
Master | Next Rank: 500 Posts
Posts: 447
Joined: 19 Apr 2009
Location: Kolkata,India
Thanked: 7 times
GMAT Score:670
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 :(

Junior | Next Rank: 30 Posts
Posts: 17
Joined: 16 Jun 2009
Location: Santa Clara,CA
GMAT Score:700
uptowngirl92 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 :(
2/3*n- drink Regular Coffee.
2/5*2/3*n- Add cream

(2/3-4/15)*n- do not add cream.

Newbie | Next Rank: 10 Posts
Posts: 4
Joined: 26 Jul 2008

Give this a thought

by niramay » Mon Jul 20, 2009 10:36 pm
You took (2/5th fraction of 2/3rd of N) to get the no. of ppl who add cream to Regular Coffee, whereas the problem expected you to get the same as 2/5N (without multiplying it with 2/3N).

Junior | Next Rank: 30 Posts
Posts: 17
Joined: 16 Jun 2009
Location: Santa Clara,CA
GMAT Score:700

Re: Give this a thought

by rajanyadav » Tue Jul 21, 2009 8:07 am
niramay wrote:You took (2/5th fraction of 2/3rd of N) to get the no. of ppl who add cream to Regular Coffee, whereas the problem expected you to get the same as 2/5N (without multiplying it with 2/3N).
.Out of these 2/5 add cream- In that case what is "out of these? "

Master | Next Rank: 500 Posts
Posts: 345
Joined: 18 Mar 2009
Location: Sao Paulo-Brazil
Thanked: 12 times
GMAT Score:660

by shibal » Tue Jul 21, 2009 9:38 am
I prefer picking numbers in this type of question. Make N=15, then 10 drink coffee and 5 don't. From those who drink coffee, 2/5 add cream ==> 4 add cream
Now go through the answer options and plug in 15 to see were you get 4 as an answer.

Junior | Next Rank: 30 Posts
Posts: 17
Joined: 16 Jun 2009
Location: Santa Clara,CA
GMAT Score:700

by rajanyadav » Tue Jul 21, 2009 9:55 am
shibal wrote:I prefer picking numbers in this type of question. Make N=15, then 10 drink coffee and 5 don't. From those who drink coffee, 2/5 add cream ==> 4 add cream
Now go through the answer options and plug in 15 to see were you get 4 as an answer.
That would work too. My point was that the question stem was not clear.
Do 2/5 out of N add cream or 2/5 out of 2/3 out of N add cream.

Master | Next Rank: 500 Posts
Posts: 345
Joined: 18 Mar 2009
Location: Sao Paulo-Brazil
Thanked: 12 times
GMAT Score:660

by shibal » Tue Jul 21, 2009 5:22 pm
it says 'out of these, 2/5 add cream.....' so you can imply that he is talking about the 3/5 that drink coffee

Junior | Next Rank: 30 Posts
Posts: 17
Joined: 16 Jun 2009
Location: Santa Clara,CA
GMAT Score:700

by rajanyadav » Tue Jul 21, 2009 6:20 pm
shibal wrote:it says 'out of these, 2/5 add cream.....' so you can imply that he is talking about the 3/5 that drink coffee
According to me out of these refers to the whole sentence preceding it.

Newbie | Next Rank: 10 Posts
Posts: 2
Joined: 24 Jul 2009

by Ajnadin » Fri Jul 24, 2009 6:22 am
shibal wrote:I prefer picking numbers in this type of question. Make N=15, then 10 drink coffee and 5 don't. From those who drink coffee, 2/5 add cream ==> 4 add cream
Now go through the answer options and plug in 15 to see were you get 4 as an answer.
this is correct if the question is about those adding cream!
however, ur way is correct and he should use 6 instead of 4 to verify his correct formula since the question is about who do not add cream..

so, 2/5 x 10 = 6
2/5 N is the right answer.

Newbie | Next Rank: 10 Posts
Posts: 2
Joined: 24 Jul 2009
uptowngirl92 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 :(
r u sure that u did a mistake :) :)

ur answer is right! enjoy! :)

Newbie | Next Rank: 10 Posts
Posts: 2
Joined: 26 Jul 2009
uptowngirl92 wrote: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?
I don't think I'm understanding the question correctly.

(a) It asks how many of the employees who drink regular coffee (a subset of all employees, spec 2/3x) do not add cream.

(b) It states that of this subset, 2/5 add cream.

(c) Therefore, 3/5 of this subset must not add cream.

The way this question is phrased makes me think that 3/5 is the answer...

If the question was asking how many employees drink regular coffee AND do not add cream, then it would be 2/5.