Answer and explanation in white color text below
方塊 5
As 甲 do not know the card, the number of the card must have at least 2 types i.e. must be either A, Q, 4, or 5
As 乙 know that 甲 won't know the card, all the cards in that type must have numbers that at least in 2 types, therefore, the type must be either heart or diamond
After 甲 know it must be heart or diamond and he then know the card, the number cannot be A and must be either Q, 4, or 5
If the type is heart, 乙 won't be able to tell whether it is Q or 4. Only because it is 5, then he can know the answer.
Recheck scenario:
甲 was told that it is a 5. Since it could be club or diamond, he does don't know which card.
乙 was told it is diamond, as both A exist in heart and diamond and 5 exist in club and diamond, he know that no matter which card it is 甲 won't be able to know.
With the above information, 甲 know it must be heart or diamond. Since he know it is a 5, so it must be diamond 5.
As explained already, 乙 know that 甲 is able to tell the card corretly only if it is a 5.
