koreye wrote:

Long way:

find mean of the 10 numbers. mean is 100. Deduct 22.4 from 100, which equals 77.6. Only two values are below this number

Short way (not sure if this is always reliable):

You have to remember that in a normally distributed sample, Approx 16% of data points are more than 1 standard deviation from the mean, and 16% are less than 1 standard deviation from the mean (i.e. 64 % are within 1 S.d, and 32% are outside 1 S.d of the mean)

16% of the sample = 16/100 * 10 = 1.6 rounded off to 2.

No Your short cut method is not correct in this question.

Its just by chance that Your answer is matching the actual answer.