There are 10 cards, each of which has two numbers, numbered , , , , , written on it, and the numbers on any two cards are not exactly identical. The 10 cards are placed in five boxes labelled , , , , , and a card with and written on it can only be placed in box or . One placement is called “good” if there are more cards in box than in each of the other boxes. Then the total number of the “good” placements is ______.
Solution
Denote the card with written on it as . It is easy to know that these 10 cards are exactly ().
Consider the “good” placements of the cards. There are 10 cards in the five boxes, so there are at least 3 cards in box 1. The only cards that will fit in box 1 are , , and .
Case 1: The 4 cards are all placed in box 1, and at this point it is no longer possible to have 4 cards in each of the remaining boxes. Therefore, no matter how the remaining 6 cards are placed, they will fit the requirements and there are “good” placements.
Case 2: There are exactly 3 of the 4 cards in box 1 and the rest of each box contains at most 2 cards.
Consider the number of placements of , , in box 1 and in box 5.
There are 8 possible ways to place the cards , , , with 6 of them being two cards placed in one of the boxes 2, 3, 4, and the remaining 2 being one card placed in each box of 2, 3, 4.
If there are two cards of , and in a box, suppose that and are in box 2, and then can only be in box 5. Therefore, box 5 already has and , so and are in boxes 3 and 4 respectively, namely, the placement of , and is unique.
If one card of , and is placed in each box of 2, 3, 4, then there are at most 2 cards in each box of 2, 3, 4. It is only necessary to make sure that there are no more than 2 cards in box 5, namely, there are 0 or 1 card of , and in box 5, and the number of the corresponding placements is .
As a result, . By symmetry, there are “good” placements in case 2.
To sum up, there is a total of “good” placements.