Where are "the following choices?" Please don't overlook sharing this info with possible helpers. Thank you.
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)
