selecting 20% of records for each user using MYSQL

Question

Issue ID	USER
1	A
2	A
3	A
4	A
5	A
6	B
7	B
8	C

 

Hi I am trying to fetch 20% of record for each user using sql but am able to fetch randon records can some one help me in fetching in % 

 

MY output should be like 

 

 

ID	USER
1	A
3	A
6	B
8	C