Which of the following choices is the standard deviation of the sample shown below here 17,18,19,20,21

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This kind of assignment can be one on a calculator; I typed in stdDev(17,18,19,20,21) an got std dev= 1.58.

working backward, the variance is the square of that:  2.5.

To find the variance:  Find the mean of the given set of numbers.  It is

 17+18+19+20+21      95
= = 19
               5                    5

and then calculate the deviations of all 5 numbers from the mean:

17-19=-2
18-19= -1
19-19=0
20-19 = 1
21-19=2
 
Then sum up the squares of these:  4+1+0+1+4 = 10
Finally, divide this result by 5-1 (one less than the # of data points):

10
= 2.50 (variance)
(5-1)