Oracle Database Discussions

hi, iam using HP-UX with oracle10G so that, i want to know the size of of database in my system, for that i can use the follwing queries is this is correct or not?
/*
1. select sum(bytes) as total_size from dba_data_files;
ANS is:
TOTAL_SIZE
----------------------
29718740992

2.select sum(bytes) as free_size from dba_free_space;
ANS is:
FREE_SIZE
----------------------
6940393472

3. select sum(bytes) as used_size from dba_segments;
ANS is:
USED_SIZE
----------------------
25215827968

i GOT like this iam not getting is this TOTAL size is 28 GB?????????????



4.select * from DBA_FREE_SPACE_COALESCED;

ANS for this ????????????????



please help me anyone......