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AMC8 2019

AMC8 2019 · Q25

AMC8 2019 · Q25. It mainly tests Basic counting (rules of product/sum), Combinations.

Alice has $24$ apples. In how many ways can she share them with Becky and Chris so that each of the three people has at least two apples?
Alice 有 24 个苹果。她有多少种方式与 Becky 和 Chris 分苹果,使得三人各至少有 2 个苹果?
(A) 105 105
(B) 114 114
(C) 190 190
(D) 210 210
(E) 380 380
Answer
Correct choice: (C)
正确答案:(C)
Solution
Note: This solution uses the non-negative version for stars and bars. A solution using the positive version of stars is similar (first removing an apple from each person instead of 2). This method uses the counting method of stars and bars (non-negative version). Since each person must have at least $2$ apples, we can remove $2*3$ apples from the total that need to be sorted. With the remaining $18$ apples, we can use stars and bars to determine the number of possibilities. Assume there are $18$ stars in a row, and $2$ bars, which will be placed to separate the stars into groups of $3$. In total, there are $18$ spaces for stars $+ 2$ spaces for bars, for a total of $20$ spaces. We can now do $20 \choose 2$. This is because if we choose distinct $2$ spots for the bars to be placed, each combo of $3$ groups will be different, and all apples will add up to $18$. We can also do this because the apples are indistinguishable. $20 \choose 2$ is $190$, therefore the answer is $\boxed{\textbf{(C) }190}$.
注意:此解使用星星与条非负版本。使用正版本的星星解类似(先从每个人那里拿走一个苹果而不是 2 个)。 此方法使用星星与条计数法(非负版本)。由于每个人必须至少有 2 个苹果,我们从总数中移除 $2*3$ 个苹果。剩余 18 个苹果,我们使用星星与条确定可能性。假设有 18 个星星一行,和 2 个条,将放置以将星星分成 3 组。总共有 18 个星星位置 + 2 个条位置,共 20 个位置。我们现在做 $20 \choose 2$。这是因为如果我们选择 2 个不同的位置放置条,每种 3 组组合都会不同,所有苹果加起来为 18。我们也可以这样做,因为苹果是不可区分的。$20 \choose 2$ 是 190,因此答案是 $\boxed{\textbf{(C) }190}$。
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