Well this is easy, but it depends on what version of flash you are using.
If you are using 5 just drag a selection over the excess frames and then right click on the selected area then click "remove frames".
If you are using MX just click frame 30 then click frame 15 , now just right click on the selection and click "remove frames".
At least if my memory serves me correctly that is how it is done
I have to many programs open at the moment to open MX up.