hello

really need help with something.

If person A makes an event once every 50 days,
and person B makes an event once every 30 days (the same event),
and person C makes an event once every 20 days (the same event)
event can happen by 1 man or 2 or even 3, it's by chance.

how often this event is likely to happen ? (once every X days kind of solution)

I think it is a tough one.
thanks for any help