What is the standard deviation of list M? (OG13)

[alex.gellatly]

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

[GMATGuruNY]

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.

The correct answer is C.