P: Jorge was upset on his birthday.
(A): (If less than 50 people showed up to Jorge's 50th birthday party, Jorge would be upset on his birthday.)
(A): (If 50 or more people showed up to Jorge's 50th birthday party, Jorge would not be upset on his birthday.)
C: Less than 50 people must have attended Jorge's 50th birthday party.
I really don't see a difference between the two assumptions....