Check GMAT Club Decision Tracker for the Latest School Decision Releases https://gmatclub.com/AppTrack

 It is currently 24 May 2017, 11:33

### 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

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

# During an experiment, some water was removed from each of 6

Author Message
TAGS:

### Hide Tags

Manager
Joined: 12 Apr 2007
Posts: 168
Followers: 1

Kudos [?]: 10 [2] , given: 0

During an experiment, some water was removed from each of 6 [#permalink]

### Show Tags

07 Jul 2007, 17:27
2
KUDOS
6
This post was
BOOKMARKED
00:00

Difficulty:

25% (medium)

Question Stats:

68% (01:39) correct 32% (00:56) wrong based on 211 sessions

### HideShow timer Statistics

During an experiment, some water was removed from each of the 6 water tanks. If the standard deviation of the volumes of water in the tanks at the beginning of the experiment was 10 gallons, what was the standard deviation of the volumes of water in the tanks at the end of the experiment?

(1) For each tank, 30% of the volume of water that was in the tank at the beginning of the experiment was removed during the experiment.

(2) The average (arithmetic mean) volume of water in the tanks at the end of the experiment was 63 gallons.

OPEN DISCUSSION OF THIS QUESTION IS HERE: during-an-experiment-some-water-was-removed-from-each-of-94166.html
[Reveal] Spoiler: OA

Last edited by Bunuel on 11 May 2014, 06:20, edited 1 time in total.
Renamed the topic, edited the question and added the OA.
Director
Joined: 12 Jun 2006
Posts: 532
Followers: 2

Kudos [?]: 123 [1] , given: 1

Re: gmatprep standard deviation DS [#permalink]

### Show Tags

08 Jul 2007, 03:41
1
KUDOS
plaguerabbit wrote:
During an experiment, some water was removed from each of 6 water tanks. If the standard deviation of the volumes of water in the tanks at the beginning of the experiment was 10 gallons, what was the standard deviation of the volumes of water in the tanks at the end of the experiment?

(1) For each tank, 30 percent of the volume of water that was in the tank at the beginning of the experiment was removed during the experiment.

(2) The average (arithmetic mean) volume of water in the tanks at the end of the experiment was 63 gallons.

OA later
A. since .3 is being removed from each tank won't SD remain the same?
Manager
Joined: 12 Apr 2007
Posts: 168
Followers: 1

Kudos [?]: 10 [0], given: 0

Re: gmatprep standard deviation DS [#permalink]

### Show Tags

08 Jul 2007, 06:12
ggarr wrote:
plaguerabbit wrote:
During an experiment, some water was removed from each of 6 water tanks. If the standard deviation of the volumes of water in the tanks at the beginning of the experiment was 10 gallons, what was the standard deviation of the volumes of water in the tanks at the end of the experiment?

(1) For each tank, 30 percent of the volume of water that was in the tank at the beginning of the experiment was removed during the experiment.

(2) The average (arithmetic mean) volume of water in the tanks at the end of the experiment was 63 gallons.

OA later
A. since .3 is being removed from each tank won't SD remain the same?

no, the concept that i missed here was that the standard deviation would decrease by .3 since the distances between the values were all decreased by 30%.

So the std dev will be 7 from S1
Manager
Joined: 15 Nov 2007
Posts: 135
Followers: 1

Kudos [?]: 54 [0], given: 2

Re: gmatprep standard deviation DS [#permalink]

### Show Tags

02 May 2008, 00:19
SVP
Joined: 04 May 2006
Posts: 1902
Schools: CBS, Kellogg
Followers: 23

Kudos [?]: 1122 [2] , given: 1

Re: gmatprep standard deviation DS [#permalink]

### Show Tags

02 May 2008, 01:06
2
KUDOS
dominion wrote:

I think the holder went to the B-school already!

My computer doesnot support the supericacl, I must write it in a very detail, i explain A only

A.
Beginning:
6 water tanks: t1, t2, t3, t4....t6
mean t =(t1+t2+t3+...+t6)/6
SD =10 = √∑(ti - t)^2 (i = 1...6)

After removing each tanhk 30% water:

6 water tanks: 0.7*t1,.....0.7*t6
new mean =(0.7*t1 + 0.7*t2 +....+0.7*t6)/6 = 0.7* t
new SD = √∑(0.7*ti - 0.7*t)^2 (i = 1...6)
new SD = 0.7*10 =7
_________________
Director
Joined: 14 Jan 2007
Posts: 775
Followers: 2

Kudos [?]: 148 [3] , given: 0

Re: gmatprep standard deviation DS [#permalink]

### Show Tags

03 May 2008, 11:09
3
KUDOS
1
This post was
BOOKMARKED
Two important properties of std dev:
1. Adding a constant to each term does not alter the std dev
2. Multiplying each term by a constant leads to the new std dev = constant * old std dev

Property 2 applies here.
GMAT Club Legend
Joined: 09 Sep 2013
Posts: 15428
Followers: 649

Kudos [?]: 207 [1] , given: 0

Re: During an experiment, some water was removed from each of 6 [#permalink]

### Show Tags

11 May 2014, 05:19
1
KUDOS
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.
_________________
Math Expert
Joined: 02 Sep 2009
Posts: 38856
Followers: 7725

Kudos [?]: 106028 [6] , given: 11602

Re: During an experiment, some water was removed from each of 6 [#permalink]

### Show Tags

11 May 2014, 06:22
6
KUDOS
Expert's post
2
This post was
BOOKMARKED
During an experiment, some water was removed from each of the 6 water tanks. If the standard deviation of the volumes of water in the tanks at the beginning of the experiment was 10 gallons, what was the standard deviation of the volumes of water in the tanks at the end of the experiment?

(1) For each tank, 30% of the volume of water that was in the tank at the beginning of the experiment was removed during the experiment.
(2) The average (arithmetic mean) volume of water in the tanks at the end of the experiment was 63 gallons.

You should know that:
If we add or subtract a constant to each term in a set:
Mean will increase or decrease by the same constant.
SD will not change.

If we increase or decrease each term in a set by the same percent (multiply all terms by the constant):
Mean will increase or decrease by the same percent.
SD will increase or decrease by the same percent.

You can check it yourself:
SD of a set: {1,1,4} will be the same as that of {5,5,8} as second set is obtained by adding 4 to each term of the first set.

That's because Standard Deviation shows how much variation there is from the mean. And when adding or subtracting a constant to each term we are shifting the mean of the set by this constant (mean will increase or decrease by the same constant) but the variation from the mean remains the same as all terms are also shifted by the same constant.

So according to this rules statement (1) is sufficient to get new SD, it'll be 30% less than the old SD so 7. As for statement (2) it's clearly insufficient as knowing mean gives us no help in getting new SD.

For more on this check Standard Deviation chapter of Math Book: math-standard-deviation-87905.html

Collection of PS questions on SD: ps-questions-about-standard-deviation-85897.html
Collection of PS questions on SD: ds-questions-about-standard-deviation-85896.html

Hope it's clear.

OPEN DISCUSSION OF THIS QUESTION IS HERE: during-an-experiment-some-water-was-removed-from-each-of-94166.html
_________________
Re: During an experiment, some water was removed from each of 6   [#permalink] 11 May 2014, 06:22
Similar topics Replies Last post
Similar
Topics:
31 Is the positive integer x divisible by each integer from 2 through 6 ? 8 23 Jul 2016, 20:17
20 A certain number of marbles are to be removed from a box con 6 12 Apr 2017, 09:03
9 Three dice, each with faces numbered from 1 through 6, were 8 01 Jun 2015, 08:01
100 During an experiment, some water was removed from each of 20 02 Jan 2017, 07:33
18 Julian bought 6 kilograms of cheese from each of Deli A and 12 07 Nov 2016, 01:03
Display posts from previous: Sort by