GMAT Question of the Day - Daily to your Mailbox; hard ones only

It is currently 20 Jun 2018, 00:32

Close

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Close

Request Expert Reply

Confirm Cancel

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

An alloy of copper and aluminum has 40% copper. An alloy of Copper and

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:

Hide Tags

1 KUDOS received
VP
VP
User avatar
D
Status: It's near - I can see.
Joined: 13 Apr 2013
Posts: 1045
Location: India
Concentration: International Business, Operations
GMAT 1: 480 Q38 V22
GPA: 3.01
WE: Engineering (Consulting)
Premium Member Reviews Badge
An alloy of copper and aluminum has 40% copper. An alloy of Copper and [#permalink]

Show Tags

New post 17 Sep 2016, 10:06
1
1
7
00:00
A
B
C
D
E

Difficulty:

  95% (hard)

Question Stats:

21% (01:36) correct 79% (03:48) wrong based on 202 sessions

HideShow timer Statistics

An alloy of copper and aluminum has 40% copper. An alloy of Copper and Zinc has Copper and Zinc in the ratio 2:7. These two alloys are mixed in such a way that in the overall alloy, there is more aluminum than Zinc, and copper constitutes x% of this alloy. What is the range of values x can take?

A) 30% ≤ x ≤ 40%

B) 32.5% ≤ x ≤ 40%

C) 32.5% ≤ x ≤ 42%

D) 33.33% ≤ x ≤ 40%

E) 33.33 % ≤ x ≤ 42%

_________________

"Success is not as glamorous as people tell you. It's a lot of hours spent in the darkness."

Current Student
avatar
B
Joined: 12 Jul 2013
Posts: 7
Re: An alloy of copper and aluminum has 40% copper. An alloy of Copper and [#permalink]

Show Tags

New post 24 Sep 2016, 11:05
GMATantidote wrote:
An alloy of copper and aluminum has 40% copper. An alloy of Copper and Zinc has Copper and Zinc in the ratio 2:7. These two alloys are mixed in such a way that in the overall alloy, there is more aluminum than Zinc, and copper constitutes x% of this alloy. What is the range of values x can take?

A) 30% ≤ x ≤ 40%

B) 32.5% ≤ x ≤ 40%

C) 32.5% ≤ x ≤ 42%

D) 33.33% ≤ x ≤ 40%

E) 33.33 % ≤ x ≤ 42%


Copper to Zinc ratio = 2/9 = 22.2 %
Hence
x - 22.2 / 40 - x = P / Q

P Ration of and according to Question P > Q . Plug in the values a) only fits
Manager
Manager
User avatar
S
Joined: 30 Dec 2015
Posts: 89
GPA: 3.92
WE: Engineering (Aerospace and Defense)
Re: An alloy of copper and aluminum has 40% copper. An alloy of Copper and [#permalink]

Show Tags

New post 02 Oct 2016, 20:09
GMATantidote wrote:
An alloy of copper and aluminum has 40% copper. An alloy of Copper and Zinc has Copper and Zinc in the ratio 2:7. These two alloys are mixed in such a way that in the overall alloy, there is more aluminum than Zinc, and copper constitutes x% of this alloy. What is the range of values x can take?

A) 30% ≤ x ≤ 40%

B) 32.5% ≤ x ≤ 40%

C) 32.5% ≤ x ≤ 42%

D) 33.33% ≤ x ≤ 40%

E) 33.33 % ≤ x ≤ 42%


X is the weighted average of \(\frac{2}{9}\) = ~20% and 40%. Since Zn < Al, weight given to 40% (Cu/Al) is more than the weight given to ~20% (Cu/Zn), which entails the average 'X' will be pulled closer to 40% than to ~20%. Also, the average will most certainly be greater than 30%, which is in the middle of 40% and 20%, but will be less than 40%.
There can be any possibilities of X, depending on the weightage given. Hence I chose (A), which takes in values, all the way from the middle of 20% & 40% to the max of 40% : i.e. from 30% to 40%
_________________

If you analyze enough data, you can predict the future.....its calculating probability, nothing more!

Expert Post
4 KUDOS received
Veritas Prep GMAT Instructor
User avatar
P
Joined: 16 Oct 2010
Posts: 8100
Location: Pune, India
Re: An alloy of copper and aluminum has 40% copper. An alloy of Copper and [#permalink]

Show Tags

New post 03 Oct 2016, 03:44
4
GMATantidote wrote:
An alloy of copper and aluminum has 40% copper. An alloy of Copper and Zinc has Copper and Zinc in the ratio 2:7. These two alloys are mixed in such a way that in the overall alloy, there is more aluminum than Zinc, and copper constitutes x% of this alloy. What is the range of values x can take?

A) 30% ≤ x ≤ 40%

B) 32.5% ≤ x ≤ 40%

C) 32.5% ≤ x ≤ 42%

D) 33.33% ≤ x ≤ 40%

E) 33.33 % ≤ x ≤ 42%


What is the source of this problem? It is certainly not GMAT specific because the numbers given are hard to work with. Also, the options are not accurate. "Range" means that we need to give the values that x does take. Option (A) covers extra values (like codomain) while option (B) doesn't cover all values. In a question like this, you cannot have an "approximate value" of x.


Given: "in the overall alloy, there is more aluminum than Zinc,"

The maximum aluminium you can have is when you mix copper and aluminium alloy (say alloy A) with infinitesimally small amount of copper and zinc alloy (alloy B). In that case, you will have almost 40% of copper in the mix (because alloy A has 40% of copper). So x% < 40%.

Minimum case:
60% of alloy A > (7/9) of alloy B
A/B > 35/27

If the two alloys are mixed in this ratio, the copper concentration will be:
Using weighted average:

Cavg = (2/5 * 35 + 2/9 * 27)/(35 + 27)

Cavg = (14 + 6)/62= 20/62 = 32.26% (rounded to 2 decimal places)

So x% > 32.26%

Answer: 32.26 < x < 40
_________________

Karishma
Veritas Prep | GMAT Instructor
My Blog

Get started with Veritas Prep GMAT On Demand for $199

Veritas Prep Reviews

1 KUDOS received
Manager
Manager
avatar
Joined: 29 Aug 2008
Posts: 110
An alloy of copper and aluminum has 40% copper. An alloy of Copper and [#permalink]

Show Tags

New post 06 Oct 2016, 10:37
1
VeritasPrepKarishma wrote:
GMATantidote wrote:
An alloy of copper and aluminum has 40% copper. An alloy of Copper and Zinc has Copper and Zinc in the ratio 2:7. These two alloys are mixed in such a way that in the overall alloy, there is more aluminum than Zinc, and copper constitutes x% of this alloy. What is the range of values x can take?

A) 30% ≤ x ≤ 40%

B) 32.5% ≤ x ≤ 40%

C) 32.5% ≤ x ≤ 42%

D) 33.33% ≤ x ≤ 40%

E) 33.33 % ≤ x ≤ 42%


What is the source of this problem? It is certainly not GMAT specific because the numbers given are hard to work with. Also, the options are not accurate. "Range" means that we need to give the values that x does take. Option (A) covers extra values (like codomain) while option (B) doesn't cover all values. In a question like this, you cannot have an "approximate value" of x.


Given: "in the overall alloy, there is more aluminum than Zinc,"

The maximum aluminium you can have is when you mix copper and aluminium alloy (say alloy A) with infinitesimally small amount of copper and zinc alloy (alloy B). In that case, you will have almost 40% of copper in the mix (because alloy A has 40% of copper). So x% < 40%.

Minimum case:
60% of alloy A > (7/9) of alloy B
A/B > 35/27

If the two alloys are mixed in this ratio, the copper concentration will be:
Using weighted average:

Cavg = (2/5 * 35 + 2/9 * 27)/(35 + 27)

Cavg = (14 + 6)/62= 20/62 = 32.26% (rounded to 2 decimal places)

So x% > 32.26%

Answer: 32.26 < x < 40


VeritasPrepKarishma


Thanks for the answer, I went through your posts on weighted average and scale on Veritasprep and really liked the approach.

Can you pls explain - In this particular question, while calculating the maximum for Aluminum how are we using the statement that Aluminum can be no more than Zinc?


TIA

Also would be interested in getting answer to this question, since I learned Weighted average from your posts - so thought it will be good to understand your POV.

a-person-can-walk-at-a-constant-rate-of-8mph-and-can-bike-at-a-rate-of-215605.html#p1744545

Thanks
Expert Post
1 KUDOS received
Veritas Prep GMAT Instructor
User avatar
P
Joined: 16 Oct 2010
Posts: 8100
Location: Pune, India
Re: An alloy of copper and aluminum has 40% copper. An alloy of Copper and [#permalink]

Show Tags

New post 06 Oct 2016, 21:17
1
gauravk wrote:
VeritasPrepKarishma wrote:
GMATantidote wrote:
An alloy of copper and aluminum has 40% copper. An alloy of Copper and Zinc has Copper and Zinc in the ratio 2:7. These two alloys are mixed in such a way that in the overall alloy, there is more aluminum than Zinc, and copper constitutes x% of this alloy. What is the range of values x can take?

A) 30% ≤ x ≤ 40%

B) 32.5% ≤ x ≤ 40%

C) 32.5% ≤ x ≤ 42%

D) 33.33% ≤ x ≤ 40%

E) 33.33 % ≤ x ≤ 42%


What is the source of this problem? It is certainly not GMAT specific because the numbers given are hard to work with. Also, the options are not accurate. "Range" means that we need to give the values that x does take. Option (A) covers extra values (like codomain) while option (B) doesn't cover all values. In a question like this, you cannot have an "approximate value" of x.


Given: "in the overall alloy, there is more aluminum than Zinc,"

The maximum aluminium you can have is when you mix copper and aluminium alloy (say alloy A) with infinitesimally small amount of copper and zinc alloy (alloy B). In that case, you will have almost 40% of copper in the mix (because alloy A has 40% of copper). So x% < 40%.

Minimum case:
60% of alloy A > (7/9) of alloy B
A/B > 35/27

If the two alloys are mixed in this ratio, the copper concentration will be:
Using weighted average:

Cavg = (2/5 * 35 + 2/9 * 27)/(35 + 27)

Cavg = (14 + 6)/62= 20/62 = 32.26% (rounded to 2 decimal places)

So x% > 32.26%

Answer: 32.26 < x < 40


VeritasPrepKarishma


Thanks for the answer, I went through your posts on weighted average and scale on Veritasprep and really liked the approach.

Can you pls explain - In this particular question, while calculating the maximum for Aluminum how are we using the statement that Aluminum can be no more than Zinc?


TIA

Also would be interested in getting answer to this question, since I learned Weighted average from your posts - so thought it will be good to understand your POV.

a-person-can-walk-at-a-constant-rate-of-8mph-and-can-bike-at-a-rate-of-215605.html#p1744545

Thanks


When calculating maximum aluminium, we are practically taking only the aluminium-copper alloy. We are mixing just an infinitesimally small amount of zinc-copper alloy. Hence, the quantity of aluminium will certainly be more than the quantity of zinc which is what we require.
_________________

Karishma
Veritas Prep | GMAT Instructor
My Blog

Get started with Veritas Prep GMAT On Demand for $199

Veritas Prep Reviews

Non-Human User
User avatar
Joined: 09 Sep 2013
Posts: 7006
Premium Member
Re: An alloy of copper and aluminum has 40% copper. An alloy of Copper and [#permalink]

Show Tags

New post 23 Jan 2018, 07:07
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________

GMAT Books | GMAT Club Tests | Best Prices on GMAT Courses | GMAT Mobile App | Math Resources | Verbal Resources

Re: An alloy of copper and aluminum has 40% copper. An alloy of Copper and   [#permalink] 23 Jan 2018, 07:07
Display posts from previous: Sort by

An alloy of copper and aluminum has 40% copper. An alloy of Copper and

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  


GMAT Club MBA Forum Home| About| Terms and Conditions and Privacy Policy| GMAT Club Rules| Contact| Sitemap

Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne

Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.