You can do this using a running total. Take a look at this article that explains how to do a running total in sql.

http://www.sqlservercentral.com/articles/T-SQL/68467/[/url]

If you get stuck feel free to post back here. It is best if you can provide data in a consumable format. Since you are brand new around here I did this for you so you can see.

;with Data(ClassID, MemberID, Weight) as

(

select 'A', 1, 10 union all

select 'A', 2, 20 union all

select 'A', 3, 40 union all

select 'A', 5, 25 union all

select 'A', 6, 15 union all

select 'A', 8, 10 union all

select 'B', 1, 50 union all

select 'B', 2, 10 union all

select 'B', 3, 60 union all

select 'B', 6, 400

)