问题描述：
SAT 2 maths
A coin is tossed three times.given that at least one head appears.what is the probability that exactly two heads appear?
Answer 3/7 为什么不是3/8?
A coin is tossed three times.given that at least one head appears.what is the probability that exactly two heads appear?
Answer 3/7 为什么不是3/8?
问题解答：
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