SD - mean

Mean is 10 and standard deviation is 3.5. If the data set contains 40 data values, approximately how many of the data values will fall within the range 6.5 to 13.5?


x =  40


Solution in text x =

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Rule 1 sigma for a normal distribution (from the average on both sides distance standard deviation sigma) lies 68.26% of the values, if are normally distributed.

Math student
please I don't get the last part of the solution


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