2010 AMC 10A 第 18 题
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18.
Bernardo 从集合 中随机选出 个不同的数,并按降序排列形成一个 位数。Silvia 从集合 中随机选出 个不同的数,也按降序排列形成一个 位数。Bernardo 的数大于 Silvia 的数的概率是多少?
Bernardo randomly picks distinct numbers from the set and arranges them in descending order to form a -digit number. Silvia randomly picks distinct numbers from the set and also arranges them in descending order to form a -digit number. What is the probability that Bernardo's number is larger than Silvia's number?
答案:B
解答:
分两种情况:Bernardo 选到 ,或没有选到这个数。
情况一:Bernardo 选到 。
由于一个数字已经固定,另外两个数有 种选法。
总选法为 ,概率为
注意,如果 Bernardo 选到 ,他一定比 Silvia 的数大。
因此 Bernardo 在这种情况下总是获胜。
情况二:Bernardo 没有选到 。
这种情况发生的概率为 。由于两人此时从相同的数字中选择,除了平局以外,双方获胜的概率相同。
Silvia 与 Bernardo 选到同一组数字的概率为 所以在这种情况下,Bernardo 的数较大的概率为
因此总概率为
所以正确答案是 B。
There are two cases: Bernardo picks a or he doesn't.
Case 1: Bernardo picks a
Since a number is fixed, there are ways to choose the other two numbers.
There are a total of ways to pick all three numbers. The probability is then
Note that if Bernardo picks a he automatically has a greater number than Silvia.
This means that Bernardo always wins in this case.
Case 2: Bernardo doesn't pick a
There is a chance of this happening. Since both people are choosing from the same numbers, they have an equal chance of winning.
We still need to find the probability that the numbers are the same. There is a chance that Silvia chooses the same numbers as Bernardo. The probability that Bernardo gets a higher number is then
The total probability of Bernardo getting a higher number is then
Thus, B is the correct answer.
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