2. 外语
  3. Select one number from the set {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}, take down its value and then put it back. Select one number from

Select one number from the set {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}, take down its value and then put it back. Select one number from

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问题 Select one number from the set {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}, take down its value and then put it back. Select one number from the set again and also take down its value. If the sum of the two values is 8, what is probability of one of them being 5?
选项 A、1/9
B、2/10
C、1/4
D、1/8
E、2/9
答案E
解析 对本题而言：分母为两个数相加和为8的所有可能取法。哪两个数相加和为8呢?(0，8)，(1，7)，(2，6)，(3，5)，(4，4)；我们知道(0，8)，(1，7)，(2，6)，(3，5)都要算作两次，而(4，4)却只能算作一次，因为(4，4)反过来还是(4，4)，所以共有4×2+1=9种取法。和为8的情况中包含数字5有2种取法，所以满足要求的取法有2种。这样所求概率为2/9，选择E。
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