Hi Vincen,

We're told that a certain set of numbers has an average (arithmetic mean) of 50 and a standard deviation of 50.5 and that two numbers (M and N) in the set are both within 2 standard deviations of the average. We're asked which of the following COULD be the sum of M and N.

Since the AVERAGE of the set of numbers is 50, two S.D.s "up" would be 50+2(50.5) = 151 and two S.D.s "down" would be 50 - 2(50.5) = -51. Thus, each of the two values (M and N) can be anything from -51 to +151 each. The smallest that the sum could be would be (2)(-51) = -102 and the largest the sum could be would be (2)(151) = +302. Four of the five answers fall outside of that range, thus there's only one answer that's possible...

Final Answer: D

GMAT assassins aren't born, they're made,
Rich

Vincen wrote:A certain set of numbers has an average (arithmetic mean) of 50 and a standard deviation of 50.5. If m and n, two numbers in the set, are both within 2 standard deviations from the average, then which of the following could be the sum of m and n?

A. -200
B. -130
C. -104
D. 51
E. 305

The OA is D.

I am really confused here. Can any expert give me some help? I would be thankful.

Two standard deviations below the mean of 50 is 50 - 2*(50.5) = 50 - 101 = -51, and two standard deviations above 50 is 50 + 2*(50.5) = 50 + 101 = 151. Thus, we have -51 < m, n < 151. We see that m + n must be greater than 2*(-51) = -102 and less than 2*151 = 302. Thus, all answer choices besides D are eliminated. Indeed, we can pick m = 0 and n = 51 so that m + n = 51.

Answer: D