The average weight of a class is x pounds. When a new stud : GMAT Problem Solving (PS)
Check GMAT Club Decision Tracker for the Latest School Decision Releases http://gmatclub.com/AppTrack

 It is currently 24 Jan 2017, 14:51

### 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

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# The average weight of a class is x pounds. When a new stud

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

### Hide Tags

Manager
Joined: 09 Feb 2013
Posts: 120
Followers: 1

Kudos [?]: 852 [5] , given: 17

The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

27 Feb 2013, 03:15
5
This post received
KUDOS
9
This post was
BOOKMARKED
00:00

Difficulty:

45% (medium)

Question Stats:

71% (05:24) correct 29% (02:55) wrong based on 184 sessions

### HideShow timer Statistics

The average weight of a class is x pounds. When a new student weighing 80 pounds joins the class, the average decreases by 1 pound. In a few months the student’s weight increases to 110 pounds and the average weight of the class becomes x + 4 pounds. None of the other students’ weights changed. What is the value of x?

A. 85
B. 86
C. 88
D. 90
E. 92
[Reveal] Spoiler: OA

_________________

Kudos will encourage many others, like me.
Good Questions also deserve few KUDOS.

Last edited by Bunuel on 27 Feb 2013, 04:27, edited 1 time in total.
Edited the question.
Math Expert
Joined: 02 Sep 2009
Posts: 36638
Followers: 7106

Kudos [?]: 93661 [6] , given: 10583

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

27 Feb 2013, 04:49
6
This post received
KUDOS
Expert's post
6
This post was
BOOKMARKED
emmak wrote:
The average weight of a class is x pounds. When a new student weighing 80 pounds joins the class, the average decreases by 1 pound. In a few months the student’s weight increases to 110 pounds and the average weight of the class becomes x + 4 pounds. None of the other students’ weights changed. What is the value of x?

A. 85
B. 86
C. 88
D. 90
E. 92

When the student weighs 80 pounds the average weight is x - 1 pounds;
When the student weighs 110 pounds the average weight is x + 4 pounds.

So, the increase in total weight of 110 - 80 = 30 pounds corresponds to the increase in average weight of (x + 4) - (x - 1) =5 pounds, which means that there are 30/5 = 6 students (including the new one). So, initially there were 5 student.

Total weight = 5x + 80 = 6(x-1) --> x = 86 pounds.

Answer: B.

Hope it's clear.
_________________
Verbal Forum Moderator
Joined: 10 Oct 2012
Posts: 630
Followers: 81

Kudos [?]: 1120 [2] , given: 136

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

27 Feb 2013, 04:15
2
This post received
KUDOS
emmak wrote:
The average weight of a class is x pounds. When a new student weighing 80 pounds joins the class, the average decreases by 1 pound. In a few months the student’s weight increases to 110 pounds and the average weight of the class becomes x + 4 pounds. None of the other students’ weights changed. What is the value of x?

85

86

88

90

92

Let the number of students by y. We know that their weight is fixed. Thus, initially, when a new student with weight 80 pounds joins,we have :

$$80+y*1 = x-1.$$

Again, when the same student weighs 110 pounds, we have :

$$110-4*y = x+4.$$

Thus, multiplying the first equation by 4 and adding both we get,

320+110 =$$5*x$$

or x = $$\frac{430}{5}$$= 86.

B.
_________________
Math Expert
Joined: 02 Sep 2009
Posts: 36638
Followers: 7106

Kudos [?]: 93661 [1] , given: 10583

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

10 Mar 2013, 05:17
1
This post received
KUDOS
Expert's post
skamal7 wrote:
bunnel,
Can you explain how Total weight = 5x + 80 = 6(x-1) --> x = 86 pounds.??

(Total weight) = (The # of students) * (The average weight) = 6*(x-1).

Similarly, the total weight for 5 students is 5x, which we know that is 80 pounds less than the total weight for 6 students.

Hope it's clear.
_________________
Intern
Joined: 12 Apr 2013
Posts: 9
Location: Germany
Concentration: Finance, Accounting
GMAT Date: 05-28-2013
GPA: 3.6
Followers: 0

Kudos [?]: 11 [1] , given: 20

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

21 Apr 2013, 09:02
1
This post received
KUDOS
gmatquant25 wrote:
vinaymimani wrote:
emmak wrote:
The average weight of a class is x pounds. When a new student weighing 80 pounds joins the class, the average decreases by 1 pound. In a few months the student’s weight increases to 110 pounds and the average weight of the class becomes x + 4 pounds. None of the other students’ weights changed. What is the value of x?

85

86

88

90

92

Let the number of students by y. We know that their weight is fixed. Thus, initially, when a new student with weight 80 pounds joins,we have :

$$80+y*1 = x-1.$$

Again, when the same student weighs 110 pounds, we have :

$$110-4*y = x+4.$$

Thus, multiplying the first equation by 4 and adding both we get,

320+110 =$$5*x$$

or x = $$\frac{430}{5}$$= 86.

B.

could anyone please elaborate on how the above two equations were derived ?

thanks ~

Hi,

Ao - old average
An - new average
X - weight of the new student
n - number of students including the new guy
c - any constant (in our case -1)

This is the equation for calculating the average in that case

$$\frac{X + (n-1)*Ao}{n}=Ao + c$$

$$X=(1-n)*Ao+n*(Ao+c)$$

$$X=Ao+n*c$$

$$X=Ao+c-c+n*c$$

(Ao+c = An)

$$X=An+c*(n-1)$$

So the weight of the new student equals the new average plus n-1(each of the old students) times c.

Btw. it is not necessary to use above trick to get the result for that problem. If anyone is interested I can post an alternative way.
_________________

........................................................................................
See it big and keep it simple.

Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 7132
Location: Pune, India
Followers: 2140

Kudos [?]: 13716 [1] , given: 222

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

21 Apr 2013, 19:59
1
This post received
KUDOS
Expert's post
1
This post was
BOOKMARKED
emmak wrote:
The average weight of a class is x pounds. When a new student weighing 80 pounds joins the class, the average decreases by 1 pound. In a few months the student’s weight increases to 110 pounds and the average weight of the class becomes x + 4 pounds. None of the other students’ weights changed. What is the value of x?

A. 85
B. 86
C. 88
D. 90
E. 92

Check out this post for a discussion on mean and how to solve such questions logically.

http://www.veritasprep.com/blog/2012/04 ... etic-mean/
_________________

Karishma
Veritas Prep | GMAT Instructor
My Blog

Get started with Veritas Prep GMAT On Demand for \$199

Veritas Prep Reviews

Verbal Forum Moderator
Joined: 02 Aug 2009
Posts: 4252
Followers: 332

Kudos [?]: 3484 [1] , given: 101

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

13 May 2016, 08:47
1
This post received
KUDOS
Expert's post
PrakharGMAT wrote:
Hi Experts / chetan2u,

Please have a look on my approach. I have created 3 equations-

1) (Sum) / N = X

2) When a new student of weight 80 is added the equations become
(Sum +80)/ (N+1) = X-1

3) When the students gain weight by 110. In this case the students have put on more weight so we just need to add 110 in the sum.
(Sum + 110) / N= X+4

After solving these equations I am getting X=102 which is not correct.
I know, I am missing something.

Can you please assist.

Thanks and Regards,
Prakhar

hi,
i find the approach to be OK BUT the highlighted portion has to be N+1 and not N..
Try putting that and check your answer
_________________

Absolute modulus :http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

Verbal Forum Moderator
Joined: 02 Aug 2009
Posts: 4252
Followers: 332

Kudos [?]: 3484 [1] , given: 101

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

13 May 2016, 08:58
1
This post received
KUDOS
Expert's post
PrakharGMAT wrote:
Hi chetan2u,

I am getting the corect answer now. But I am not able to understand, the question says In a few months the student’s weight increases to 110 pounds.
So, the students have gain weight, then why do we need to denominator as (N+1).

Can you please assist..?

Thanks

Prakhar ,
if you read the Q,it says the person who joined at 80 kgs has increased to 110 kgs..
and Including him TOTAL is N+1..
hope it is clear..
Quote:
When a new studentweighing 80 pounds joins the class, the average decreases by 1 pound. In a few months the student’s weight increases to 110 pounds and the average weight of the class becomes x + 4 pounds.

_________________

Absolute modulus :http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

Senior Manager
Joined: 02 Sep 2012
Posts: 259
Location: United States
Concentration: Entrepreneurship, Finance
GMAT Date: 07-25-2013
GPA: 3.83
WE: Architecture (Computer Hardware)
Followers: 5

Kudos [?]: 180 [0], given: 99

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

09 Mar 2013, 10:56
bunnel,
Can you explain how Total weight = 5x + 80 = 6(x-1) --> x = 86 pounds.??
_________________

"Giving kudos" is a decent way to say "Thanks" and motivate contributors. Please use them, it won't cost you anything

Intern
Joined: 27 Mar 2013
Posts: 37
Followers: 0

Kudos [?]: 110 [0], given: 37

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

18 Apr 2013, 12:24
vinaymimani wrote:
emmak wrote:
The average weight of a class is x pounds. When a new student weighing 80 pounds joins the class, the average decreases by 1 pound. In a few months the student’s weight increases to 110 pounds and the average weight of the class becomes x + 4 pounds. None of the other students’ weights changed. What is the value of x?

85

86

88

90

92

Let the number of students by y. We know that their weight is fixed. Thus, initially, when a new student with weight 80 pounds joins,we have :

$$80+y*1 = x-1.$$

Again, when the same student weighs 110 pounds, we have :

$$110-4*y = x+4.$$

Thus, multiplying the first equation by 4 and adding both we get,

320+110 =$$5*x$$

or x = $$\frac{430}{5}$$= 86.

B.

could anyone please elaborate on how the above two equations were derived ?

thanks ~
Verbal Forum Moderator
Joined: 10 Oct 2012
Posts: 630
Followers: 81

Kudos [?]: 1120 [0], given: 136

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

18 Apr 2013, 14:01
1
This post was
BOOKMARKED
Quote:

Let the number of students by y. We know that their weight is fixed. Thus, initially, when a new student with weight 80 pounds joins,we have :

$$80+y*1 = x-1.$$

Again, when the same student weighs 110 pounds, we have :

$$110-4*y = x+4.$$

Thus, multiplying the first equation by 4 and adding both we get,

320+110 =$$5*x$$

or x = $$\frac{430}{5}$$= 86.

B.

could anyone please elaborate on how the above two equations were derived ?

Quote:
thanks ~

When a new student joins and this results in a drop by 1 in the average, it is as if each student present in the class gave 1 pound to him. Also, after getting 1 pound from each student, the new weight the student has must equal the new average. Thus, 80+y*1 = x-1

Similarly, when his weight becomes 110 pounds, to increase the average , he must have contributed 4 pounds to each student .Just as above, this new weight must equal the average. Thus, 110-4*y = x+4
_________________
Senior Manager
Joined: 23 Oct 2010
Posts: 386
Location: Azerbaijan
Concentration: Finance
Schools: HEC '15 (A)
GMAT 1: 690 Q47 V38
Followers: 21

Kudos [?]: 326 [0], given: 73

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

21 Apr 2013, 20:46
1
This post was
BOOKMARKED
(x+4)-(x-1)=5
110-80=30

30/5=6

80+6=86
_________________

Happy are those who dream dreams and are ready to pay the price to make them come true

I am still on all gmat forums. msg me if you want to ask me smth

Manager
Status: Tougher times ...
Joined: 04 Nov 2012
Posts: 51
Location: India
GMAT 1: 480 Q32 V25
WE: General Management (Manufacturing)
Followers: 2

Kudos [?]: 26 [0], given: 44

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

21 Apr 2013, 23:15
emmak wrote:
The average weight of a class is x pounds. When a new student weighing 80 pounds joins the class, the average decreases by 1 pound. In a few months the student’s weight increases to 110 pounds and the average weight of the class becomes x + 4 pounds. None of the other students’ weights changed. What is the value of x?

A. 85
B. 86
C. 88
D. 90
E. 92

lets assume Total weight = T ; Total number of people = N and average = X (as given in question )

as per the average formula .. X=T / N

Condition 1 : ( X-1 ) = ( T+80 )/ ( N+1) ... => X-N=81

Condition 2 : (X+4) = ( T+110) /(N+1)... => X+ 4N = 106

Solving we get X=86 ..!
_________________

Kabilan.K
Kudos is a boost to participate actively and contribute more to the forum

Current Student
Joined: 04 Mar 2013
Posts: 69
Location: India
Concentration: Strategy, Operations
Schools: Booth '17 (M)
GMAT 1: 770 Q50 V44
GPA: 3.66
WE: Operations (Manufacturing)
Followers: 5

Kudos [?]: 52 [0], given: 27

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

22 Apr 2013, 01:15
Bunuel wrote:
emmak wrote:
The average weight of a class is x pounds. When a new student weighing 80 pounds joins the class, the average decreases by 1 pound. In a few months the student’s weight increases to 110 pounds and the average weight of the class becomes x + 4 pounds. None of the other students’ weights changed. What is the value of x?

A. 85
B. 86
C. 88
D. 90
E. 92

When the student weighs 80 pounds the average weight is x - 1 pounds;
When the student weighs 110 pounds the average weight is x + 4 pounds.

So, the increase in total weight of 110 - 80 = 30 pounds corresponds to the increase in average weight of (x + 4) - (x - 1) =5 pounds, which means that there are 30/5 = 6 students (including the new one). So, initially there were 5 student.

Total weight = 5x + 80 = 6(x-1) --> x = 86 pounds.

Answer: B.

Hope it's clear.

This is the best way to solve this.
_________________

When you feel like giving up, remember why you held on for so long in the first place.

GMAT Club Legend
Joined: 09 Sep 2013
Posts: 13548
Followers: 578

Kudos [?]: 163 [0], given: 0

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

21 Oct 2014, 14:53
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 Club Legend
Joined: 09 Sep 2013
Posts: 13548
Followers: 578

Kudos [?]: 163 [0], given: 0

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

12 Dec 2015, 23:22
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.
_________________
Manager
Joined: 12 Jan 2015
Posts: 234
Followers: 8

Kudos [?]: 31 [0], given: 79

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

13 May 2016, 08:40
Hi Experts / chetan2u,

Please have a look on my approach. I have created 3 equations-

1) (Sum) / N = X

2) When a new student of weight 80 is added the equations become
(Sum +80)/ (N+1) = X-1

3) When the students gain weight by 110. In this case the students have put on more weight so we just need to add 110 in the sum.
(Sum + 110) / N = X+4

After solving these equations I am getting X=102 which is not correct.
I know, I am missing something.

Can you please assist.

Thanks and Regards,
Prakhar
_________________

Thanks and Regards,
Prakhar

Manager
Joined: 12 Jan 2015
Posts: 234
Followers: 8

Kudos [?]: 31 [0], given: 79

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

13 May 2016, 08:54
Hi chetan2u,

I am getting the corect answer now. But I am not able to understand, the question says In a few months the student’s weight increases to 110 pounds.
So, the students have gain weight, then why do we need to denominator as (N+1).

Can you please assist..?

Thanks
_________________

Thanks and Regards,
Prakhar

Manager
Joined: 12 Jan 2015
Posts: 234
Followers: 8

Kudos [?]: 31 [0], given: 79

Re: The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

13 May 2016, 09:50
Hi chetan2u,

Thanks a lot for clearing my doubt.
_________________

Thanks and Regards,
Prakhar

Director
Joined: 07 Dec 2014
Posts: 502
Followers: 3

Kudos [?]: 82 [0], given: 2

The average weight of a class is x pounds. When a new stud [#permalink]

### Show Tags

13 May 2016, 18:32
let w=total original weight of students
n=number of students
x=w/n=average weight of original students
as w increases from 80 to 110, or 30 pounds, x increases from x-1 to x+4, or 5 pounds
w/x=30/5=n=6 students
equation 1: 6(x-1)=w+80
equation 2: 5x=w
subtracting e2 from e1,
x=86 pounds
The average weight of a class is x pounds. When a new stud   [#permalink] 13 May 2016, 18:32

Go to page    1   2    Next  [ 22 posts ]

Similar topics Replies Last post
Similar
Topics:
The average (arithmetic mean) weight of 5 friends is 150 pounds. If Jo 3 31 Oct 2016, 02:15
2 The average weight of a group of 30 friends increases by 1 kg when the 6 13 Mar 2016, 11:40
1 The weight of four dogs is determined to be 25 pounds, 31 pounds, 35 3 29 Jul 2015, 14:44
8 Four packages have an average weight of 10.5 pounds. What is the minim 8 17 Jul 2015, 00:15
1 S represents the sum of the weights of n fish in pounds. 5 13 Nov 2008, 04:40
Display posts from previous: Sort by

# The average weight of a class is x pounds. When a new stud

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

 Powered by phpBB © phpBB Group and phpBB SEO 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®.