Calculating the Standard Deviation

a) easy
b) In the formula you use n if you have ALL the observations of the population (you could get that after doing infinite measures). If you have a sample of the population, you should use n-1.
Practically: when you have a small number of observations (a small sample), you use n-1. If you have a large sample (for example 200 measures) then you could use either n or n-1 since it makes small difference.
So, in your case you use the formula:

c) I can't understand this one... Does it mean that all observations will have the same value? If that's the case, then the standard deviation should be 0. But it's very hard for every measure to have the same value...


And something else: Try to write down the questions if possible, so that other people with the same questions will be able to find them.

Mean means the average, so just add them all up and divide by the number of observations.

Thanking you kindly for the help!