Finding which file is in which package

[jbarnes8]

The question isn't exaclty debian specific, but the answer may be. Basically, I spend way to much time trying to find which files are in certain packages, For instance, just yesterday I installed opera without any errors. But when I ran it, it said it could not locate libqt-mt.so.3 (or something similar to that). I spent 30 minutes trying to figure out which package it was in so that i could use 'apt-get'.
I finally found a possible package after carefully searching through 'aptitude' but I wasn't sure it was the right one. Luckily it was.

Anyway, any advice? I'm still pretty new to linux, so things that may seem obvious to you aren't so obvious to me. (For example, i'm not sure about naming conventions. In this example, I think knowledge of that would have proved useful.)

I'm sure this information is out there somewhere, but there's a million other things that I'm trying to learn at the same time. If you can't give me a fish, just tell me which lake to fish in

[Zelmo]

Probably the easiest way it to go to packages.debian.org and search for a package's contents.