It may not happen often, but from time to time you’ll end up with a 25fps sequence in which you need to ingest clips that have been shot at 24fps and at 25fps. In Final Cut Pro X you can mix different clip formats and frame rates voluntarily, but you won’t have control over the end result. Other than just drop clips in a Final Cut Pro X sequence (aka Timeline) and not worry about it, there are five ways to retime a 24fps clip to a 25fps one.
The first retiming trick is using the Retime Editor in Final Cut Pro X. With your 24fps clip loaded in your 25fps Timeline, just select the Retime Editor and drag the slider to 104%. This will speed up your 24fps clip to 25fps, including its audio. Pitch will slightly increase too.
The rest of this brief tutorial discusses the second way you’ll have to work using Final Cut Pro X as well as covers other apps besides Final Cut Pro X.