This seems to work according to what you have posted...

SQL> with raw_data as (
2 select rownum + 10000 as r
3 from dual
4 connect by rownum < 10
5 )
6 , elements as (
7 select r
8 , trunc(r,-1) as base_number
9 , remainder(r,trunc(r,-1)) as remain
10 from raw_data
11 )
12 select r as original
13 , base_number + case
14 when remain between 0 and 2
15 then 0
16 when remain between 3 and 5
17 then 5
18 when remain between 6 and 8
19 then 6
20 when remain = 9
21 then 10
22 end as adjusted
23 from elements;
ORIGINAL ADJUSTED
---------- ----------
10001 10000
10002 10000
10003 10005
10004 10005
10005 10005
10006 10006
10007 10006
10008 10006
10009 10010
10010 10010
10 rows selected.

So we can try for other size integers and ranges...

SQL> with raw_data as (
2 select rownum + 1000 as r
3 from dual
4 connect by rownum < 100
5 )
6 , elements as (
7 select r
8 , trunc(r,-1) as base_number
9 , remainder(r,trunc(r,-1)) as remain
10 from raw_data
11 where r between 1040 and 1050
12 )
13 select r as original
14 , base_number + case
15 when remain between 0 and 2
16 then 0
17 when remain between 3 and 5
18 then 5
19 when remain between 6 and 8
20 then 6
21 when remain = 9
22 then 10
23 end as adjusted
24 from elements;
ORIGINAL ADJUSTED
---------- ----------
1040 1040
1041 1040
1042 1040
1043 1045
1044 1045
1045 1045
1046 1046
1047 1046
1048 1046
1049 1050
1050 1050
11 rows selected.

SQL> with raw_data as (
2 select rownum + 1234000 as r
3 from dual
4 connect by rownum < 100
5 )
6 , elements as (
7 select r
8 , trunc(r,-1) as base_number
9 , remainder(r,trunc(r,-1)) as remain
10 from raw_data
11 where r between 1234050 and 1234060
12 )
13 select r as original
14 , base_number + case
15 when remain between 0 and 2
16 then 0
17 when remain between 3 and 5
18 then 5
19 when remain between 6 and 8
20 then 6
21 when remain = 9
22 then 10
23 end as adjusted
24 from elements;
ORIGINAL ADJUSTED
---------- ----------
1234050 1234050
1234051 1234050
1234052 1234050
1234053 1234055
1234054 1234055
1234055 1234055
1234056 1234056
1234057 1234056
1234058 1234056
1234059 1234060
1234060 1234060
11 rows selected.

Regards

Adrian