I need help. (Obviously)

How can I find simple (and weighted) averages of a column of numbers... but not the entire column. Example, I have a column of 50 data points and want to average the last 8 numbers only. The trick here is that my data set is 50 columns wide as well, but each successive column gets one number less. So Column A has 50, from row 1 to 50, Column b has row 1 to 49 etc. all the way to one row only out at column 50. I want to average the last 8 numbers only and then drag my formula across all columns and not have to go into the formula bar and change my array by one row for each column.

Does this make sense? If anyone is an actuary I am working on insurance loss development triangles if that helps. I am trying to make getting my LDF picks easier.

Thanks,

Jason