Add range of values under a column using rowid
Technically, you could use Oracle's SUM function (good explanation here: http://www.techonthenet.com/oracle/functions/sum.php). Your code would look something like this:

select sum(time)/3600
  from table_name
 where rownum &gt;= 1 and rownum &lt;=5;


However, without knowing more about the table, I don't know if the TIME column is simply an integer, or is a date/timestamp field. If it is date/timestamp, SUM won't really do what you need it to. 

One other word of caution, using rownum doesn't always return the same results between runs. Please take a look at http://www.oracle.com/technetwork/issue-archive/2006/06-sep/o56asktom-086197.html for a much more succinct explanation than I can provide.