Slow down a video 10 times

Hi.

I have a few 240fps videos that I want to slow down to 24fps. Virtualdub doesn't open these mp4 files. I've tried Avidemux but it only goes up to 200fps, and I can't seem to slow it down, maybe it's just me doing it the wrong way.
I also have Freemake Video Converter but I haven't found a way to slow it down there.

Do you think Avidemux can do it? Or should I try something else?

Thank you in advance!