Author 
Message 
TAGS:

Hide Tags

Manager
Joined: 13 Aug 2009
Posts: 107
WE 1: 4 years in IT

The list shown consists of the times, in seconds, that it took each of
[#permalink]
Show Tags
Updated on: 15 Oct 2019, 06:51
Question Stats:
64% (02:07) correct 36% (02:16) wrong based on 553 sessions
HideShow timer Statistics
70, 75, 80, 85, 90, 105, 105, 130, 130, 130 The list shown consists of the times, in seconds, that it took each of 10 schoolchildren to run a distance of 400 on of meters. If the standard devastation of the 10 running times is 22.4 seconds, rounded to the nearest tenth of a second, how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running times? A. one B. two C. three D. four E. five
Official Answer and Stats are available only to registered users. Register/ Login.
Originally posted by raghavs on 28 Mar 2010, 10:42.
Last edited by Bunuel on 15 Oct 2019, 06:51, edited 4 times in total.
OA added




Math Expert
Joined: 02 Sep 2009
Posts: 60647

Re: The list shown consists of the times, in seconds, that it took each of
[#permalink]
Show Tags
23 Jan 2012, 04:01
karthiksms wrote: Could someone please explain this? thanks 70 75 80 85 90 105 105 130 130 130
The list shown consist of the times, in seconds, that it took each of 10 school children to run a distance of 400 meter. If the SD of ten running times is 22.4 seconds, rounded to nearest tenth of second, how many of the 10 running times are more than one SD below the mean of the 10 running times?A. one B. two C. three D. four E. five "How many of the 10 running times are more than one SD below the mean" means how many data points from given 10 are less than mean1SD. We are given that SD=22.4, so we should find mean > mean=100 > there are only 2 data points below 10022.4=77.6, namely 70 and 75. Answer: B. Also discussed here: 7075808590105105130130130thelistshownconsistof100361.htmlSimilar problems: mathquestionsmeanscore88502.html?hilit=below%20mean%20deviationstatisticsquestion99221.html?hilit=below%20mean%20deviation#p764887Hope it helps.
_________________




Manager
Joined: 30 Aug 2009
Posts: 216
Location: India
Concentration: General Management

Re: The list shown consists of the times, in seconds, that it took each of
[#permalink]
Show Tags
28 Mar 2010, 12:13
raghavs wrote: standard deviation..first question stumped me the mean of the 10 running times is 100 sec. we need to find how many of the recorded times are less than 1 SD below the mean i.e how many took less than 100 sec  1SD = 100 sec  22.4 sec = 77.6sec. The times are 70 and 75. hence 2



Manager
Joined: 01 Feb 2010
Posts: 171

Re: The list shown consists of the times, in seconds, that it took each of
[#permalink]
Show Tags
28 Mar 2010, 22:35
raghavs wrote: standard deviation..first question stumped me mean is 99 SD is 22.4 One SD block away = 9922.4 = 76.6 so there are two numbers less than mean which are One SD block away 70 & 75 hence TWO.



Manager
Status: DDay is on February 10th. and I am not stressed
Affiliations: American Management association, American Association of financial accountants
Joined: 12 Apr 2011
Posts: 154
Location: Kuwait
Schools: Columbia university

Re: The list shown consists of the times, in seconds, that it took each of
[#permalink]
Show Tags
23 Jan 2012, 07:57
first add the values and devide by 10 to get the mean of 100. then butract 22.4 from the 100.....> 10022.4=76.6
check how many numbers in the list are less than 76.6! there are two numbers
hope this helps



Math Expert
Joined: 02 Sep 2009
Posts: 60647

Re: The list shown consists of the times, in seconds, that it took each of
[#permalink]
Show Tags
03 Jun 2013, 03:09
Bumping for review and further discussion*. Get a kudos point for an alternative solution! *New project from GMAT Club!!! Check HERE
_________________



Manager
Joined: 27 Apr 2012
Posts: 57
Location: United States
GMAT Date: 06112013
GPA: 3.5
WE: Marketing (Consumer Products)

Re: The list shown consists of the times, in seconds, that it took each of
[#permalink]
Show Tags
03 Jun 2013, 05:15
The first step here is to calculate the mean :
(70+75+80+85+90+105+105+130+130+130)/10 = 100
SD = 22.4,
one SD below the mean = (100 – 22.4) = 77.6
Only 2 values 70 and 75 are less than 77.6, hence B is the answer



Intern
Joined: 26 Mar 2013
Posts: 11
Location: United States

Re: The list shown consists of the times, in seconds, that it took each of
[#permalink]
Show Tags
03 Jun 2013, 16:23
To find the mean, take a wild stab and pick a number within the range provided by the given numbers. Now, find the delta between that number and each number in the set. So now you get the overall delta from your "assumed mean". Split that up equally among each number. So adding that to your "assumed mean" will give you the actual mean.
I took 100 and found the difference between it and each of the numbers in the set. Turns out that the overall delta neatly becomes 0. So 100 is the actual mean.



Manager
Joined: 28 Feb 2012
Posts: 103
Concentration: Strategy, International Business
GPA: 3.9
WE: Marketing (Other)

Re: The list shown consists of the times, in seconds, that it took each of
[#permalink]
Show Tags
04 Jun 2013, 04:39
70 75 80 85 90 105 105 130 130 130
The list shown consist of the times, in seconds, that it took each of 10 school children to run a distance of 400 meter. If the SD of ten running times is 22.4 seconds, rounded to nearest tenth of second, how many of the 10 running times are more than one SD below the mean of the 10 running times? A. one B. two C. three D. four E. five
The most time consuming part in this question is to define the mean. Under exam pressure and time pressure it is very easy to make mistake. it is easier to group numbers: 130*3=390; 105*2=210; 75+85=160; 70+80=150; 90; Next stage combine results, again using more convenient ways to calculate: 390+210=600; 160+150=310; 90. 600+310+90=1000. Since there are 10 numbers the mean is 100. Questions asks to find the quantity of numbers one SD BELOW the mean, which is 10022,4=77,6. There are only two numbers below 77,6. The answer is B



Manager
Joined: 07 Apr 2012
Posts: 87
Location: United States
Concentration: Entrepreneurship, Operations
GPA: 3.9
WE: Operations (Manufacturing)

Re: The list shown consists of the times, in seconds, that it took each of
[#permalink]
Show Tags
04 Sep 2013, 00:55
ziko wrote: 70 75 80 85 90 105 105 130 130 130
The list shown consist of the times, in seconds, that it took each of 10 school children to run a distance of 400 meter. If the SD of ten running times is 22.4 seconds, rounded to nearest tenth of second, how many of the 10 running times are more than one SD below the mean of the 10 running times? A. one B. two C. three D. four E. five
The most time consuming part in this question is to define the mean. Under exam pressure and time pressure it is very easy to make mistake. it is easier to group numbers: 130*3=390; 105*2=210; 75+85=160; 70+80=150; 90; Next stage combine results, again using more convenient ways to calculate: 390+210=600; 160+150=310; 90. 600+310+90=1000. Since there are 10 numbers the mean is 100. Questions asks to find the quantity of numbers one SD BELOW the mean, which is 10022,4=77,6. There are only two numbers below 77,6. The answer is B Pardon me for my understanding, but "are <more than> <one SD below the mean> of the 10 running times? So isn't it asking values greater than 77.6 ?



Math Expert
Joined: 02 Sep 2009
Posts: 60647

Re: The list shown consists of the times, in seconds, that it took each of
[#permalink]
Show Tags
04 Sep 2013, 02:49
ygdrasil24 wrote: ziko wrote: 70 75 80 85 90 105 105 130 130 130
The list shown consist of the times, in seconds, that it took each of 10 school children to run a distance of 400 meter. If the SD of ten running times is 22.4 seconds, rounded to nearest tenth of second, how many of the 10 running times are more than one SD below the mean of the 10 running times? A. one B. two C. three D. four E. five
The most time consuming part in this question is to define the mean. Under exam pressure and time pressure it is very easy to make mistake. it is easier to group numbers: 130*3=390; 105*2=210; 75+85=160; 70+80=150; 90; Next stage combine results, again using more convenient ways to calculate: 390+210=600; 160+150=310; 90. 600+310+90=1000. Since there are 10 numbers the mean is 100. Questions asks to find the quantity of numbers one SD BELOW the mean, which is 10022,4=77,6. There are only two numbers below 77,6. The answer is B Pardon me for my understanding, but "are <more than> <one SD below the mean> of the 10 running times? So isn't it asking values greater than 77.6 ? WRONG reading: How many of the 10 running times are more than one SD below the mean. So more than MeanSD. CORRECT reading: How many of the 10 running times are more than one SD below the mean. So less than MeanSD.
_________________



Manager
Joined: 07 Apr 2012
Posts: 87
Location: United States
Concentration: Entrepreneurship, Operations
GPA: 3.9
WE: Operations (Manufacturing)

Re: The list shown consists of the times, in seconds, that it took each of
[#permalink]
Show Tags
04 Sep 2013, 03:13
Bunuel wrote: ygdrasil24 wrote: ziko wrote: 70 75 80 85 90 105 105 130 130 130
The list shown consist of the times, in seconds, that it took each of 10 school children to run a distance of 400 meter. If the SD of ten running times is 22.4 seconds, rounded to nearest tenth of second, how many of the 10 running times are more than one SD below the mean of the 10 running times? A. one B. two C. three D. four E. five
The most time consuming part in this question is to define the mean. Under exam pressure and time pressure it is very easy to make mistake. it is easier to group numbers: 130*3=390; 105*2=210; 75+85=160; 70+80=150; 90; Next stage combine results, again using more convenient ways to calculate: 390+210=600; 160+150=310; 90. 600+310+90=1000. Since there are 10 numbers the mean is 100. Questions asks to find the quantity of numbers one SD BELOW the mean, which is 10022,4=77,6. There are only two numbers below 77,6. The answer is B Pardon me for my understanding, but "are <more than> <one SD below the mean> of the 10 running times? So isn't it asking values greater than 77.6 ? WRONG reading: How many of the 10 running times are more than one SD below the mean. So more than MeanSD. CORRECT reading: How many of the 10 running times are more than one SD below the mean. So less than MeanSD. Why couldnt they have written it as How many of the 10 running times are less than one SD below the mean. This is frustrating for me



Manager
Joined: 02 Jun 2016
Posts: 59
Location: United States (MA)
GMAT 1: 770 Q51 V44
GPA: 2.4

Re: The list shown consists of the times, in seconds, that it took each of
[#permalink]
Show Tags
13 Jul 2016, 18:24
Instead of wasting valuable time adding up all 10 numbers, you can spot some easy clues the GMAT leaves behind. The first 5 numbers are perfectly symmetric around 80, so you know mean of the first 5 is 80. The second half of numbers can be quickly added together 3*130 = 390 and 105*2 = 210 for a sum of 600. Divide by 5 to get 120. Since the first half has a mean of 80 and second half has a mean of 120, and both halves have equal weight (they both represent 5 integers), the average of both halves will be right in the middle at 100. From there figuring out the answer is business as usual. Sent from my SMG928T using GMAT Club Forum mobile app



Target Test Prep Representative
Status: Founder & CEO
Affiliations: Target Test Prep
Joined: 14 Oct 2015
Posts: 9142
Location: United States (CA)

Re: The list shown consists of the times, in seconds, that it took each of
[#permalink]
Show Tags
19 Oct 2018, 15:23
raghavs wrote: 70, 75, 80, 85, 90, 105, 105, 130, 130, 130 The list shown consists of the times, in seconds, that it took each of 10 schoolchildren to run a distance of 400 on of meters. If the standard devastation of the 10 running times is 22.4 seconds, rounded to the nearest tenth of a second, how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running times?
A. one B. two C. three D. four E. five The average (mean) is: (70 + 75 + 80 + 85 + 90 + 105 + 105 + 130 +130 +130)/10 = 1000/10 = 100 Thus, 1 standard deviation below the mean is 100  22.4 = 77.6, so there are 2 running times that are more than 1 standard deviation below the mean. Answer: B
_________________
5star rated online GMAT quant self study course See why Target Test Prep is the top rated GMAT quant course on GMAT Club. Read Our Reviews If you find one of my posts helpful, please take a moment to click on the "Kudos" button.



NonHuman User
Joined: 09 Sep 2013
Posts: 14012

Re: The list shown consists of the times, in seconds, that it took each of
[#permalink]
Show Tags
03 Dec 2019, 19:12
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.
_________________




Re: The list shown consists of the times, in seconds, that it took each of
[#permalink]
03 Dec 2019, 19:12






