average data by referred cell

[deli]

i want to average the data from the cell but referred to the number of group from different cell like below

group_no value
1 20
1 23
1 23
1 21
1 22
2 24
2 25
2 27
2 29

i want the answer in continuous of one cell
i try this IF(A1:A9=1,B1:B9)), but it will only give the answer for group 1, i have about hundred of group no to handle.
kindly help me please.
tq

 

[AlphaFrog]

Excel Workbook
	A	B	C	D	E
1	group_no	value		Group	Group Avg
2	1	20		1	21.8
3	1	23		2	26.25
4	1	23
5	1	21
6	1	22
7	2	24
8	2	25
9	2	27
10	2	29
Sheet

 

[deli]

actually i want the continuous answer in column c,
means the answer i can put besides the data,
i need the command that can give continues calculate the average for a different group no but still in column c