HMMT 十一月 2011 · THM 赛 · 第 4 题

英文解析

4. [ 6 ] Toward the end of a game of Fish, the 2 through 7 of spades, inclusive, remain in the hands of threedistinguishable players: DBR, RB, and DB, such that each player has at least one card. If it is knownthat DBR either has more than one card or has an even-numbered spade, or both, in how many wayscan the players’ hands be distributed?
   Answer: 450 First, we count the number of distributions where each player has at least 1 card.
   The possible distributions are:
   • Case 1: 4/1/1: There are 3 choices for who gets 4 cards, 6 choices for the card that one ofthe single-card players holds, and 5 choices for the card the other single-card player holds, or
   3 × 6 × 5 = 90 choices.
   ( )
   • Case 2: 3/2/1: There are 6 choices for the single card, = 10 choices for the pair of cards,5
   and 3! = 6 choices for which player gets how many cards, for a total of 6 × 10 × 6 = 360 choices.2
   ( ) ( )
   6 4
   • Case 3: 2/2/2: There are = 15 choices for the cards DBR gets, = 6 for the cards that
   2 2
   RB gets, and DB gets the remaining two cards. This gives a total of 15 × 6 = 90 choices.
   Thus, we have a total of 90 + 360 + 90 = 540 ways for the cards to be distributed so that each personholds at least one.
   Next, we look at the number of ways that the condition cannot be satisfied if each player has at leastone card. Then, DBR must have no more than one card, and cannot have an even spade. We only careabout cases where he has a non-zero number of cards, so he must have exactly 1 odd spade. Then, wesee that there are 2 − 2 = 30 ways to distribute the other 5 cards among RB and DB so that neither 5
   has 0 cards. Since there are 3 odd spades, this gives 3 × 30 bad cases, so we have 540 − 90 = 450 oneswhere all the problem conditions hold.