Assuming only 365 days in a year --
1) Jack's birthday can be any day: 365/365
Mike's birthday must be the same day: 1/365
Final probability: (365/365) x (1/365)
2) The professor's birthday can be any day.
The first student can have a birthday any day except for the professor's: 364/365
The next student cannot match either of the two previous persons: 363/365
Etc.
Final probability: (364/365) x (363/365) x (362/365) x ... x (315/365)
3) This one is a guess:
The first person's birthday can be any day.
To have only two persons with the same birthday, 48 persons cannot match:
(364/365) x (363/365) x ... x (317/365)
The one that matches: (49/365)
and this person can be any of the 49:
Final probability: (364/365) x (363/365) x ... x (317/365) x (49/365) x 49
But this is just a guess.