Hi,

 a mean score on a standardized test is 50 with a standard deviation of 10.

(a)What scores 'x' fall between -1 and +1 standard deviation? 40 < x <60

(b) What percent of all scores fall between -1 and +1 standard deviation? ~68%

(c) What scores falls at +2 standard deviations? 

(d)What percentage of scores falls between +1 and +2 standard deviation? 95.4-68.2 = 27.2%

For the normal distribution: 

one  standard deviation from the mean accounts for about % of the set 

two standard deviations from the mean account for about %

and three standard deviations from the mean account for about %.

 

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