Standard deviation: hours worked by ER nurses

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Hi, Im having trouble figuring out how to do this question (when there's 2 columns of things)

The nurses' union collects data on the hours worke by operating-room nurses at the Statsville General Hospital.
datachart.jpg

Determine the mean and standard deviation for the nurses' hours.

I figured out the mean by multiplying each row by each other:
(12* 160 * 245 * 304* 210)/ 26 = 35.8

But I don't know how to the standard deviation part: I'm just not sure about this part of the forumula: xi- 35.8, what is the xi in this case?

I hope you can help, thanks
 
If you are going to use the sum of squared deviations, then the xi are the hours per week. However, you have to multiply the squares by ni, the number of employees, so you add up ni(xi-35.8)^2 and divide by 26-1. Then take the square root.
 
okay thanks, but how come you divide it by 26-1? instead of 26?
 
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