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For a certain exam,a score of 58 was 2 standard deviations b [#permalink]
06 Mar 2012, 03:06

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81% (02:03) correct
19% (00:54) wrong based on 315 sessions

For a certain exam,a score of 58 was 2 standard deviations below mean and a score of 98 was 3 standard deviations above mean.What was the mean score for the exam?

Re: For a certain exam,a score of 58 was 2 standard deviations b [#permalink]
06 Mar 2012, 03:11

5

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MaithiliGokarn wrote:

For a certain exam,a score of 58 was 2 standard deviations below mean and a score of 98 was 3 standard deviations above mean.What was the mean score for the exam?

A. 74 B. 76 C. 78 D. 80 E. 82

Welcome to GMAT Club. Below is a solution to your question.

A score of 58 was 2 standard deviations below the mean --> 58 = Mean - 2d A score of 98 was 3 standard deviations above the mean --> 98 = Mean + 3d

Re: For a certain exam,a score of 58 was 2 standard deviations b [#permalink]
14 Aug 2013, 09:37

Bunuel wrote:

MaithiliGokarn wrote:

For a certain exam,a score of 58 was 2 standard deviations below mean and a score of 98 was 3 standard deviations above mean.What was the mean score for the exam?

A. 74 B. 76 C. 78 D. 80 E. 82

Welcome to GMAT Club. Below is a solution to your question.

A score of 58 was 2 standard deviations below the mean --> 58 = Mean - 2d A score of 98 was 3 standard deviations above the mean --> 98 = Mean + 3d

Solving above for Mean = 74.

Answer: A.

Can someone please elaborate as how is this solved?

Re: For a certain exam,a score of 58 was 2 standard deviations b [#permalink]
14 Aug 2013, 10:35

1

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Expert's post

patternpandora wrote:

Bunuel wrote:

MaithiliGokarn wrote:

For a certain exam,a score of 58 was 2 standard deviations below mean and a score of 98 was 3 standard deviations above mean.What was the mean score for the exam?

A. 74 B. 76 C. 78 D. 80 E. 82

Welcome to GMAT Club. Below is a solution to your question.

A score of 58 was 2 standard deviations below the mean --> 58 = Mean - 2d A score of 98 was 3 standard deviations above the mean --> 98 = Mean + 3d

Solving above for Mean = 74.

Answer: A.

Can someone please elaborate as how is this solved?

Please elaborate what exactly didn't you understand. Thank you.

Re: For a certain exam,a score of 58 was 2 standard deviations b [#permalink]
17 Apr 2014, 07:59

patternpandora wrote:

Bunuel wrote:

MaithiliGokarn wrote:

For a certain exam,a score of 58 was 2 standard deviations below mean and a score of 98 was 3 standard deviations above mean.What was the mean score for the exam?

A. 74 B. 76 C. 78 D. 80 E. 82

Welcome to GMAT Club. Below is a solution to your question.

A score of 58 was 2 standard deviations below the mean --> 58 = Mean - 2d A score of 98 was 3 standard deviations above the mean --> 98 = Mean + 3d

Solving above for Mean = 74.

Answer: A.

Can someone please elaborate as how is this solved?

you can either solve as Bunuel did or exploit number properties and plug in from the answer choices:

58 = Mean - 2d -----> 2d = Mean - 58 98 = Mean + 3d -----> 3d = 98 - Mean

Thus your d must be both a multiple of 2 (2n) or a multiple of 3 (2n+1). Giving a fast glance at the answer choices, since E-E = E, the first equation is satisfied in every answer choice. The second equation, however, is satisfied in two answers only, respectively A and D.

Now, since d is a unique value for both equations, calculating d must yield the same result. It does but for one choice only and that choice is A.

\(d=\frac{74-58}{2}=\frac{98-74}{3}\)

This is intended to be a back-up method, for sure not as fast as that one Bunuel came up with but when you're under pressure everything that pops up in your mind is useful. _________________

learn the rules of the game, then play better than anyone else.

Re: For a certain exam,a score of 58 was 2 standard deviations b [#permalink]
17 Apr 2014, 18:19

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MaithiliGokarn wrote:

For a certain exam,a score of 58 was 2 standard deviations below mean and a score of 98 was 3 standard deviations above mean.What was the mean score for the exam?

A. 74 B. 76 C. 78 D. 80 E. 82

These are concepts of normal distribution. You don't need to delve much into details but you must understand what 1 SD, 2 SD etc mean.

Say you have a distribution of weights in a class of children. The mean weight of the class is, say 46 kgs. If I tell you that 1 standard deviation is of 5 kgs, it means that 1 SD below mean is 5 kgs less than 46 kgs which is 41 kgs. This also means that 34% children have weight ranging from 46 kgs to 41 kgs but you don't have to worry about that.

So 2 SD below mean will be 2*5 = 10 kgs less than 46 i.e. 36 kgs.

Similarly, 3 SD above mean will be 3*5 = 15 kgs more than 46 i.e. 61 kgs.

In the original question, you are given that 58 is 2 SD below mean and 98 is 3 SD above mean. So from 58 to 98, we have 5 SDs. \(\frac{{98 - 58}}{5} = 8\) = SD

Since SD is 8 so mean is at \(58 + 2*8 = 74\) _________________

Re: For a certain exam,a score of 58 was 2 standard deviations b [#permalink]
13 Sep 2014, 18:38

Every list of numbers has a mean. every number on the list has a deviation from the mean: that is how far that number is from the mean. deviation from the mean = (value) – (mean) Or Value= Mean - Deviation 58 = M - 2d 98 = M - 3d Solve it M = 74

Re: For a certain exam,a score of 58 was 2 standard deviations b [#permalink]
08 Dec 2014, 15:44

For those confused like myself, 58=M-2d and 98=M+3d is a system of equations. We want the value of M, so isolate D in one of the equations. then plug that value of d into the other equation and solve for M.

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