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  3. There are 20 classes in School Y. The number of students in each class is either

There are 20 classes in School Y. The number of students in each class is either

游客2024-01-12  11
问题 There are 20 classes in School Y. The number of students in each class is either 27 or 28. Now the school plans to set one extra class such that the average number of students in each class is 0.5 less than before. What is the number of students in the extra class? ________.
选项
答案 17
解析 设20个班平均学生数为X有27＜X＜28，原来的总人数为20X。而新加的班的学生数是21(X-0.5)-20X，化简后为X=10.5。因为27＜X＜28，所以16.5＜X-10.5＜17.5。又因为人数必须是整数，所以答案是17。
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