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58,60,62,64,66,68,70,72,74,76,78,80,82,84,86,88

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Joined: 07 Oct 2016
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58,60,62,64,66,68,70,72,74,76,78,80,82,84,86,88 [#permalink]

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New post 28 Nov 2016, 09:26
4
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A
B
C
D
E

Difficulty:

  95% (hard)

Question Stats:

42% (01:09) correct 58% (02:00) wrong based on 66 sessions

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58,60,62,64,66,68,70,72,74,76,78,80,82,84,86,88

The number of students in each of the 10 different classes is a different number from the above list. What is the standard deviation of the number of students in 10 classes.

1. the Mean number of students is equal to the median number of students in the the 10 classes.
2. number of students in any class is more than 63 and the avg. number of students in 10 classes is same as average of the above list
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Re: 58,60,62,64,66,68,70,72,74,76,78,80,82,84,86,88 [#permalink]

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New post 30 Nov 2016, 11:41
Can someone explains why A is not suff? Is that because if mean=avg I can not says that the numbers are equally spread? (the condition works only on the other way around, right?)
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Re: 58,60,62,64,66,68,70,72,74,76,78,80,82,84,86,88 [#permalink]

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New post 03 Dec 2016, 03:55
gmatdemolisher1234 wrote:
58,60,62,64,66,68,70,72,74,76,78,80,82,84,86,88

The number of students in each of the 10 different classes is a different number from the above list. What is the standard deviation of the number of students in 10 classes.

1. the Mean number of students is equal to the median number of students in the the 10 classes.
2. number of students in any class is more than 63 and the avg. number of students in 10 classes is same as average of the above list



Please explain how statement B helps find standard deviation
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58,60,62,64,66,68,70,72,74,76,78,80,82,84,86,88 [#permalink]

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New post 17 Feb 2017, 13:19
S-2) Given Students aregreater than 63. So possible range is from 64 to 88. Avg of given set of original numbers is 73. So to keep this average, we need to eliminate numbers 84 to 88. So we have elements of set {64 to 82}. Now we can find the numbers and SD. Sufficient.

S-1) I thought Mean = Median so equidistance elements. So the sequence of elements can be starting 58 or any number SD will remain same. This is because adding constant C, Standard Deviation doesnot change. But the answer says only B. Cant I infer the set as Equidistance ? Bunnel or Any one kindly help ?
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Re: 58,60,62,64,66,68,70,72,74,76,78,80,82,84,86,88 [#permalink]

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New post 17 Feb 2017, 20:07
gmatdemolisher1234 wrote:
58,60,62,64,66,68,70,72,74,76,78,80,82,84,86,88

The number of students in each of the 10 different classes is a different number from the above list. What is the standard deviation of the number of students in 10 classes.

1. the Mean number of students is equal to the median number of students in the the 10 classes.
2. number of students in any class is more than 63 and the avg. number of students in 10 classes is same as average of the above list




Math experts please provide an explanation on how to tackle such questions.
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Re: 58,60,62,64,66,68,70,72,74,76,78,80,82,84,86,88 [#permalink]

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New post 17 Feb 2017, 20:11
2
coolkl wrote:
S-2)
S-1) I thought Mean = Median so equidistance elements. So the sequence of elements can be starting 58 or any number SD will remain same. This is because adding constant C, Standard Deviation doesnot change. But the answer says only B. Cant I infer the set as Equidistance ? Bunnel or Any one kindly help ?


Hi,

There are 16 numbers with mean in middle of 72&74 or 73..
Let's see the statements..

1. the Mean number of students is equal to the median number of students in the the 10 classes.
mean=median does not necessarily mean equidistant..
Example..
Five numbers with average 74..
1) 70,72,74,76,78.... Equidistant
2) 66,72,74,78,80... Now these are not Equidistant..
So the standard deviation will be different for both above examples
Hence insufficient

2. number of students in any class is more than 63 and the avg. number of students in 10 classes is same as average of the above list
This tells us that the lowest number can be 64 and average is 73..
Only possibility is the numbers are 64,66,68,70,72 and corresponding numbers will be 74,76,78,80,82
Hence the standard deviation will always be same
Sufficient

B
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Re: 58,60,62,64,66,68,70,72,74,76,78,80,82,84,86,88   [#permalink] 17 Feb 2017, 20:11
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