Solution:
In counting the number of sets of 3 cards, we first want to choose which of our two attributes will be good and which of our two attributes will not be good. There are (24)=6 such choices.
Now consider the two attributes which are not good, attribute X and attribute Y. Since these are not good, some value should appear exactly twice. Suppose the value a appears twice and b appears once for attribute X and that the value c appears twice and d appears once for attribute Y. There are three choices for a and then two choices for b; similarly, there are three choices for c and then two choices for d. This gives 3⋅2⋅3⋅2=36 choices of a,b,c, and d.
There are two cases to consider. The first is that there are two cards which both have a and c, while the other card has both b and d. The second case is that only one card has both a and c, while one card has a and d and the other has b and c.
Case 1:
The three cards need to be distinct. Card 3 is necessarily distinct from Card 1 and Card 2, but we need to ensure that Card 1 and Card 2 are distinct from each other. There are
9 choices for the two good attributes of Card 1, and then
8 choices for the two good attributes of Card 2. But we also want to divide by
2 since we do not care about the order of Card 1 and Card 2. So there are
29⋅8=36 choices for the good attributes on Card 1 and Card 2. Then, the values of the good attributes of Card 1 and Card 2 uniquely determine the values of the good attributes of Card 3.
Case 2:
Card 1, Card 2, and Card 3 will all be distinct no matter what the values of the good attributes are, because the values of attributes
X and
Y are unique to each card. So there are
9 possibilities for the values of the good attributes on Card 1, and then there are
9 more possibilities for the values of the good attributes on Card 2. We do not have to divide by
2 this time, since Card 1 and Card 2 have distinct values in
X and
Y. So there are
92=81 possibilities here.
So our final answer is 6⋅62⋅(36+81)=25272.