I have to average out some numbers for each group then print out a total. example for each transaction (this is the detail part) there is one amount

Depending on the ave number of days, the amount must be place in the corresponding column.

Trans Days Amount (1 to 20 days) (21 to 30) (31 to 40)
1 10 1000
1 30
Ave 20 1000

2 30 2000
2 40
Ave 35 2000

2 20 2000
2 20
Ave 20 3000

Total 4000 2000

The problem is how to get the total for each column. Any suggestions?
Is there a way to know if a group has changed? So that when i know it's a new group i can save the Ave days in a variable and add the totals

TIA