SQL & PL/SQL

I would like based on this query add a column that will display the time elapsed between the 'eventtime' and the 'time_received:

select a.transaction_id, e.eventcode, e.eventtime, a.time_received
from request_history_tbl a 
   join eventlog_tbl e on a.transaction_id = e.transaction_id
   where e.eventtime >= sysdate - 30
   and e.eventcode = '50'
   order by 3 desc;

the time_received and eventtime are of type date.
how can i add a new column per instance 'elapsed that display the diff in seconds per instance between eventtime and time_received?