20. 4月30日是全国交通安全反思日,学校将举行交通安全知识竞赛,第一轮选拔共设有
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四个问题,规则如下:①每位参加者计分器的初始分均为10分,答对问题
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分别加1分,2分,3分,6分,答错任一题减2分;②每回答一题,计分器显示累计分数,当累计分数小于8分时,答题结束,淘汰出局;当累计分数大于或等于14分时,答题结束,进入下一轮;当答完四题,若累计分数仍不足14分时,答题结束,淘汰出局,若累计分数大于或等于14分时,答题结束,进入下一轮;③每位参加者按问题
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顺序作答,直至答题结束.假设甲同学对问题
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回答正确的概率依次为
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,且各题回答正确与否相互之间没有影响.