Omar recorded the number of hours he worked each week for a year. Below is a random sample that he took from his data. 13, 17, 9, 21 What is the standard deviation for the data?

mean =  60 / 4 = 15

13 - 15 = -2
17 - 15 =    2
9 - 15 = -6
21 - 15 = 6
Total of squares  =  4 + 4 + 36 + 36  =  80

because this is a sample we divide this by n - 1 ( = 3)

Standard deviation  =  sqrt (80/3)  = 5.16 answer