I would like to do the following calculation. Say in column A I have values
in A1 to A10. in cell B10 I would like to do the following calculation :

[(A10*1)+(A9*2)+(A8*3)]/[A10+A9+A8] in B9 it would be
[(A9*1)+(A8*2)+(A7*3)]/[A90+A8+A7] etc.

For the above, the the period is 3 (A10 to A8 and A9 to A7), I would also
like to have a variable to change the number of periods. If it is 5 then use
A10 to A6 etc.