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Register now and save up to $200 Available with Beat the GMAT members only code • Free Practice Test & Review How would you score if you took the GMAT Available with Beat the GMAT members only code • 1 Hour Free BEAT THE GMAT EXCLUSIVE Available with Beat the GMAT members only code • Free Veritas GMAT Class Experience Lesson 1 Live Free Available with Beat the GMAT members only code ## List M(not shown) consists of 8 different integers, each of This topic has 2 expert replies and 0 member replies eitijan Master | Next Rank: 500 Posts Joined 30 Jan 2016 Posted: 157 messages Thanked: 1 times #### List M(not shown) consists of 8 different integers, each of Thu Mar 03, 2016 6:09 am Elapsed Time: 00:00 • Lap #[LAPCOUNT] ([LAPTIME]) 4, 6, 8f 10,12, 14, 16,18, 20, 22 List M(not shown) consists of 8 different integers,each of which is in the list shown. What is the standard deviation of the numbers in list M? (1) The average (arithmetic mean) of the numbers in list M is equal to the average of the numbers in the list shown. (2) List Mdoes not contain 22. For the list of 10 nos, mean = Mean of 8 terms (M) = 13 Statement 1: Here, S(n terms)= n/2{2a+(n-1)d} where, n= no of terms in the list a= first term of the list d= constant difference of the term with the succeeding term Mean= Sum/n 13= (8/2[2a + 7*2])/8 This gives value of a=6 Also, S(n terms) = n/2[a+l] where, l = last term of the list 13= 8/2[6+l]/8 gives l =20 i.e. we know the list M and so Statement 1 is sufficient. Please correct me if I am wrong as OA is not A but C. Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums! ### GMAT/MBA Expert DavidG@VeritasPrep Legendary Member Joined 14 Jan 2015 Posted: 2301 messages Followed by: 115 members Thanked: 1070 times GMAT Score: 770 Thu Mar 03, 2016 6:33 am eitijan wrote: 4, 6, 8, 10,12, 14, 16,18, 20, 22 List M(not shown) consists of 8 different integers,each of which is in the list shown. What is the standard deviation of the numbers in list M? (1) The average (arithmetic mean) of the numbers in list M is equal to the average of the numbers in the list shown. (2) List Mdoes not contain 22. For the list of 10 nos, mean = Mean of 8 terms (M) = 13 Statement 1: Here, S(n terms)= n/2{2a+(n-1)d} where, n= no of terms in the list a= first term of the list d= constant difference of the term with the succeeding term Mean= Sum/n 13= (8/2[2a + 7*2])/8 This gives value of a=6 Also, S(n terms) = n/2[a+l] where, l = last term of the list 13= 8/2[6+l]/8 gives l =20 i.e. we know the list M and so Statement 1 is sufficient. Please correct me if I am wrong as OA is not A but C. The problem with this approach is that the formula is for evenly spaced sets, but we don't know if M is evenly spaced. All we know is that the mean is 13. So you could have the set: 6, 8, 10,12, 14, 16,18, 20. (If it's evenly spaced.) But you could also have: 4, 8, 10,12, 14, 16,18, 22. These sets will have different standard deviations. So the statement alone is not sufficient. _________________ Veritas Prep | GMAT Instructor Veritas Prep Reviews Save$100 off any live Veritas Prep GMAT Course

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Thu Mar 03, 2016 8:27 am
Quote:
4, 6, 8, 10, 12, 14, 16, 18, 20, 22

List M(not shown) consists of 8 different integers, each of which is in the list shown(above). What is the standard deviation of the numbers in list M?

(1) The average (arithmetic mean) of the numbers in list M is equal to the average of the numbers in the list shown.
(2) List M does not contain 22.

Thanks
The list above contains 10 integers.
List M contains 8 of these integers.
To determine the standard deviation of list M, we need to know WHICH 2 INTEGERS from the list above are NOT included in list M.

For any set of evenly spaced numbers:
average = median = (biggest+smallest)/2.
sum = number*average.

Thus, for the list of 10 integers above:
Average = (4+22)/2 = 13.
Sum = 10*13 = 130.

Statement 1: The average (arithmetic mean) of the numbers in list M is equal to the average of the numbers in the list shown.
Thus, the average of the 8 integers in list M = 13.
Sum = 8*13 = 104.
Since the sum of the 8 integers in list M (104) is 26 less than the sum of the 10 integers in the list above (130), the 2 integers NOT included in list M must have a sum of 26.
Thus, any of the following could be the pair NOT included in list M:
4+22, 6+20, 8+18, 10+16, 12+14.
INSUFFICIENT.

Statement 2: List M does not contain 22.

No way to determine the OTHER integer not included in list M.
INSUFFICIENT.

Statements 1 and 2 combined:
Since the 2 integers not included in list M must have a sum of 26, and 22 is one of these integers, the other integer not included in list M must be 4.
SUFFICIENT.

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