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题库 / OG2017Q-PS-88
The average (arithmetic mean) length per film for a group of 21 films is t minutes. If a film that runs for 66 minutes is removed from the group and replaced by one that runs for 52 minutes, what is the average length per film, in minutes, for the new group of films, in terms of t ?
t+~$2 \over 3$~
t-~$2 \over 3$~
21t+14
t+~$3 \over 2$~
t-~$3 \over 2$~
题目分析：
根据 21 个电影的平均值是 t,则 21 个电影的时间总和是 21t.去掉一个 66 分钟的电影，变成 52 分钟的电影。那么现在新的 21 个电影的时间和就是 21t-66+52=21t-14.新的 group的平均值就是(21t-14)/21=t-~$\frac{2}{3}$~.
综上：答案就是 B。
66/21-54/21
So=21t
Sn=21t-66+52=21t-14
T=Sn/21=(21t-14)/21=t-2/3