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  3. Five pairs of socks are of different color. If two socks are to be selected at random, what is the probability that the two sock

Five pairs of socks are of different color. If two socks are to be selected at random, what is the probability that the two sock

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问题 Five pairs of socks are of different color. If two socks are to be selected at random, what is the probability that the two socks will be one pair?
选项 A、1/9
B、1/2
C、5/18
D、1/5
E、2/5
答案A
解析 分母为所有可能的取法，五双袜子其实就是10只，从中取2只，一共有取法：C103=45。分子为满足要求的取法，取出的2只袜子要能够成双，必然从成双的袜子中取，一共有5双袜子，那么一共有取法C51=5。所以所求概率为：5/45=1/9，选择A。
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