[William Roman Jr] “since the sequence is 60p and has 24p footage in itas well as 60p how does it play w/o problems? “
It works because FCP is inserting extra frames to the 24p footage to make it play back properly in a 60p sequence.
The technique used for displaying 24fps footage in a 30fps video signal is called 3:2 pulldown, or “telecine.” It’s how film has been recorded on video for years. Prior to the availability of HD cameras that could record native 24p footage on memory cards, cameras recorded 24p over a 29.97 interlaced signal by adding 3:2 pulldown. If you were to step through a clip shot in this manner one frame at a time, you would see a pattern of three whole progressive frames followed by two “split” interlaced frames. For a detailed explanation of the process, check out this link:
https://www.adamwilt.com/24p/index.html#24pRecording
In 720p there is inherently no interlacing, so the process is done using duplicate frames. If you drop 24p footage in a 60p sequence, then step through it frame-by-frame, you will notice a repeating pattern of 3 identical frames followed by 2 identical frames in a repeating cadence.
Final Cut Pro has the ability to automatically add 3:2 progressive pulldown to 24p footage in a 60p sequence. However, It does not have the ability to do this properly with interlaced footage, either 29.97 SD or 1080i HD. If you try to put native 24p footage into a 29.97 SD sequence or a 1080i60 HD sequence, FCP will simply add a duplicate frame, which results in a stuttering look. This is known as 2:2:2:4 pulldown, and is visually unacceptable.
If native 24p footage is going to be placed in a 29.97 SD or 1080i60 HD sequence, proper 3:2 pulldown (interlaced) must be added in another application such as Compressor or After Effects.