Answer:
Let {A, B, C, D} be the set of boxes of regular sodas and {a, b, c, d} be the set of boxes of diet sodas. Denote [tex]_MC_P[/tex] the numbers of combinations of M boxes taken P boxes. Note that the specific combination, for example, AB from the set of boxes of regular sodas is the same as the combination BA. The number of ways to pick 2 boxes from each category is [tex]_4C_2[/tex]. Hence, the number of ways of picking 4 boxes in which he pick 2 boxes from each category is [tex]_4C_2[/tex] * [tex]_4C_2[/tex] = 6*6 = 36.
