General :: Sed - Csv Averages On Command Line - Take The Average Of All Rows With The Same 1st Number(key)

I have a big file full of records like this

1, 2, 4, 5, 6
1, 3, 5, 6, 3
1, 4, 5, 6, 6
2, 4, 5, 5, 5
2, 3, 4, 5, 2

anyway i need to take the average of all rows with the same 1st number(key). i.e.

1, 3, 4.66, 5.66, 5
2, 3.5, 4.5, 5, 3.5

I know this is something awk/sed would be great for, i just dont have enough experience with them to accomplish it, Also, what about averaging those columns together? so, after I output this to a file, id like to get another like:

1, 4.58
1, 4.125

The number of colums to add might not always be 4 either. EDIT: this might be easier to do in gnuplot, so I mainly just need an answer to the first part.