Three students are not in the same class. A is on duty every four days, B is on duty every five days and C is on duty every day. Last time they were on duty on the same day on Monday. Please calculate how many days they can be on duty on the same day. What day is today?

The least common multiple of 3, 4 and 5 is 60.

So they need at least 60 days to be on duty on the same day, that is, Friday.