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Director
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Re: 70 75 80 85 90 105 105 130 130 130 The list shown consist of [#permalink]

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New post 04 Jan 2013, 05:53
Bunuel wrote:
mainhoon wrote:
How many of the 10 running times are more than one SD below the mean.

So then 1SD below the mean is 77.6. It asks how many of the 10 running times are more than 77.6? I just replaced the one SD below the mean by the value. There are more than 2... What am I missing?


You have a problem with wording.

WRONG reading:
How many of the 10 running times are more than one SD below the mean. So more than Mean-SD.

CORRECT reading:
How many of the 10 running times are more than one SD below the mean. So less than Mean-SD.


I am still not getting this..
Why is it less and not more..
Please elaborate ..
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New post 04 Jan 2013, 05:59
Sachin9 wrote:
Bunuel wrote:
mainhoon wrote:
How many of the 10 running times are more than one SD below the mean.

So then 1SD below the mean is 77.6. It asks how many of the 10 running times are more than 77.6? I just replaced the one SD below the mean by the value. There are more than 2... What am I missing?


You have a problem with wording.

WRONG reading:
How many of the 10 running times are more than one SD below the mean. So more than Mean-SD.

CORRECT reading:
How many of the 10 running times are more than one SD below the mean. So less than Mean-SD.


I am still not getting this..
Why is it less and not more..
Please elaborate ..


This is more verbal issue than math. In English the first reading is not correct while the second one is.
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New post 04 Jan 2013, 09:13
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Got it finally..
here: http://www.beatthegmat.com/beat-the-sta ... tml#548282
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Re: 70 75 80 85 90 105 105 130 130 130 The list shown consist of [#permalink]

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New post 14 Aug 2014, 12:11
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How many of the 10 running times are more than one SD below the mean.?

This is a pretty mean and twisted question, especially on GMAT because only one out of ten would be able to crack it under 2 mins.
To understand this question, draw or visualize the standard deviation-mean graph. Look at it this way:
How many of the running times are farther than one standard deviation below the mean? Another way of looking at it is, how many values lie towards the left of one standard deviation below the mean.

This is what the graph looks like : https://www.google.co.in/url?sa=i&rct=j ... 9705195344

Hope it is clear.
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New post 25 Jul 2015, 09:22
Btw, isn't the mean in the list above 97.5? (90+105/2)
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New post 06 Oct 2015, 03:03
Long way:

find mean of the 10 numbers. mean is 100. Deduct 22.4 from 100, which equals 77.6. Only two values are below this number

Short way (not sure if this is always reliable):

You have to remember that in a normally distributed sample, Approx 16% of data points are more than 1 standard deviation from the mean, and 16% are less than 1 standard deviation from the mean (i.e. 64 % are within 1 S.d, and 32% are outside 1 S.d of the mean)

16% of the sample = 16/100 * 10 = 1.6 rounded off to 2.
Re: 70 75 80 85 90 105 105 130 130 130 The list shown consist of   [#permalink] 06 Oct 2015, 03:03

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