gre 数学题目 求解答George earned 80, 85 and 90 on the first 3 tests in his geography class. Because he didn't have time to study, he decides to guess randomly on the final test. That test has true-false questions, each worth 10 points. what is

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gre 数学题目 求解答George earned 80, 85 and 90 on the first 3 tests in his geography class. Because he didn't have time to study, he decides to guess randomly on the final test. That test has true-false questions, each worth 10 points. what is

gre 数学题目 求解答George earned 80, 85 and 90 on the first 3 tests in his geography class. Because he didn't have time to study, he decides to guess randomly on the final test. That test has true-false questions, each worth 10 points. what is
gre 数学题目 求解答
George earned 80, 85 and 90 on the first 3 tests in his geography class. Because he didn't have time to study, he decides to guess randomly on the final test. That test has true-false questions, each worth 10 points. what is the probability his final average is more than 85?

gre 数学题目 求解答George earned 80, 85 and 90 on the first 3 tests in his geography class. Because he didn't have time to study, he decides to guess randomly on the final test. That test has true-false questions, each worth 10 points. what is
如果他最后平均分向上85的话,他最后一次的分数要高于85（因为前三次的平均分就是85）,那么也就是说他最后一次必须对9个或者10个全对
因为他是随便猜的嘛,所以能猜对9个的概率是10*0.5^10（0.5^10是9次猜对、1次猜错的概率,前面那个10是,那个猜错的那次,在10道题里有10种可能）
他全猜对的概率是0.5^10
所以他能上90的概率是11*0.5^10

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