How to subtract 2 dates in oracle to get in hour and minute

I want to subtract 2 dates and represent the result in hour and minute in one decimal figure.
I have the following table and i am doing it in this way but the result is not as desired.
There is some slight variation, im sure this is simple arithmetic but im not getting it right.
select start_time, end_time, (end_time-start_time)*24 from come_leav;
START_TIME END_TIME (END_TIME-START_TIME)*24
------------------- ------------------- ------------------------
21-06-2011 14:00:00 21-06-2011 16:55:00 2.9166667
21-06-2011 07:00:00 21-06-2011 16:50:00 9.8333333
21-06-2011 07:20:00 21-06-2011 16:30:00 9.1666667
I want the result (end_time-start_time) as below.
16:55- 14:00 = 2.55
16:50-07:00 = 9.5
16:30-7:20 = 9.1 and so on.
Please help. Any help will be much appreciated x 1000 times.
Regards,
Mack