Ford Motor Company had realized returns of 15%, 25%, 25%, and 15% over four quarters.
What is the quarterly standard deviation of returns for Ford calculated from this sample?

And replacing we got:

And now we can calculate the variance like this:

And the deviation is just the square root of the variance and we got:

Explanation:

For this case we have the following data:

15%, 25%, 25%, and 15%

So for this case n = 4

And we can calculate the sample variance with the following formula:

First we need to calculate the mean with this formula:

And replacing we got:

And now we can calculate the variance like this:

And the deviation is just the square root of the variance and we got: