I have a formula field to calculate the days between order taken and last time it was sent for closure. There are several dates for closure and only the last one in the table is relevant so the formula is:

1)maximum ({table.field}, table.id}) - {date created}

then I want:
2) total closed in given time range - can be down on report itself
3) count of days < 20 and the percentage of total
4) count of days > 20 and < 50 and the percentage of total

When I try to use the formula with maximum I am unable to do a count. What can I do to calculate #3 and # 4 above.

Thanks much