Standard deviation is the square root of variance (a fact you almost certainly will not need to know on the GMAT), so the standard deviation here is 10, two standard deviations is thus 20, and 60 and 100 are the two values that are two standard deviations away from the mean of 80. Only 60 is an answer choice, so it is correct.

AAPL wrote:Economist GMAT

Teachers in United States high schools teach an average (arithmetic mean) of 80 hours per month, with a variance of 100. which of the following numbers of monthly teaching hours is exactly two standard deviations away from the mean?

A. 60
B. 74
C. 78
D. 82
E. 90

OA A

We are given that teachers in United States high schools teach an average (arithmetic mean) of 80 hours per month, with a variance of 100. We need to determine which answer choice is two standard deviations from the mean.

Recall that the standard deviation is equal to the square root of the variance.

Since the variance is 100, the standard deviation is √100 = 10, and thus two standard deviations is 10 x 2 = 20. Two standard deviations above the mean yields 80 + 20 = 100. Two standard deviations below the mean is 80 - 20 = 60. Only the value 60 is given as an answer choice.

Answer: A