询问：陈向东模拟题2中的18题

missparkUID: 3146915

题目是 George earned 80,85 and 90 on the first three tests in his geotaphy class.Because he didn't have time to study,he decides to guess randomly on the final test.That test has ten true-false questions,each worth 10 points.What is the probability his final average is more than 85?
(A)12/256
(B)11/256
(C)12/1024
(D)11/1024
(E)0.5
答案是选D 可是分母1024没问题 分子的11是怎么得来的？ 纠结纠结，求解答！

苏梳眠UID: 2717864

因为有11种情况保证average more than 85，也就是90和100.其中90的有10种情况，100的一种情况，加起来就是11中情况了。分母就是2的十次方。

missparkUID: 3146915

原来如此！！太感谢了~~~