Grouping Results In 5 Minute Increments

I need to gather data in 5 minute clusters to perform a 5 minute average report. I've been trying to forulate a query using TRUNC and GROUP BY, but the problem I'm running into is that I can't figure out how to make TRUNC operate on intervals between minutes and hours. All I see in the documentation is TRUNC(somedate, 'HH') and TRUNC(somedate, 'MI'). I've tried dividing the trunc'd dates, but I get incompatible data type errors.

I'm pretty new to PL/SQL and I'm sure this type of thing has been done before, but I'm pretty much at my wits end.

Any help/advice would be appreciated.

TIA!