1 month before
The solution looks fine.I hope you are clear with the additional step now. But sometimes, to twist the question even further, the number of observations is changed. E.g., the number of values of the variable can be n-1 or n+1 etc. instead of n. Consequently, you will divide the sum up to n-1 or n+1 terms by n-1 or n+1 respectively. So be careful about that as well.