2. 外语
  3. There are five cards marked 1, 2, 3, 4, 5 in a box. If the sum of two cards which are taken from those cards randomly (no replac

There are five cards marked 1, 2, 3, 4, 5 in a box. If the sum of two cards which are taken from those cards randomly (no replac

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问题 There are five cards marked 1, 2, 3, 4, 5 in a box. If the sum of two cards which are taken from those cards randomly (no replacement) is even. What is the probability that these two cards are both odd?
选项 A、1/4
B、3/4
C、3/10
D、3/5
E、1/3
答案B
解析 本题求的是取出两数和为偶数的情况下，两数都是奇数的概率。而不是说取出两数，两数都是奇数的概率。注意，由于取出数是no replacement，因此是“无放回的取法”，取出的两个数不相同。在偶数：奇数+奇数的情况中，共有1，3，5三个奇数，取出两个，C32=3种取法。在偶数：偶数+偶数的情况中，共有2，4两个奇数，取出两个，只有1种取法。因此所求概率是3/(3+1)=3/4。
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