CREATE TABLE #sample
(
id int identity ,
weight float 
)

insert into #sample
select 10 union all
select 30 union all
select 20 union all
select 50.5 union all
select 100 union all
select 81 

select * from #sample

id	weight   avg_current_prevoius     
1	10       10
2	30       20
3	20       20
4	50.5     35.2500 
5	100      67.225
6	81       74.312500       

;WITH tempavg
     AS (SELECT id,
                weight,
                weight avg_current_prevoius
         FROM   #sample
         WHERE  id = 1
         UNION ALL
         SELECT a.id,
                a.weight,
                .5 * ( a.weight + b.avg_current_prevoius ) 
         FROM   #sample a
                INNER JOIN tempavg b
                        ON b.id + 1 = a.id)
SELECT *
FROM   tempavg;