2015 AMC 8 第 22 题

先试着解答 2015 AMC 8 第 22 题,然后核对你的答案与视频与精心整理的解答,解答来自 LIVE by Po-Shen Loh。你也可以参加完整限时模拟考试、查看全部 2015 AMC 8 解答,或核对答案。

所有题目均经美国数学协会(MAA)官方合法授权使用。

22.

六月 11 日,一组学生排成若干行,每行 1515 人。六月 22 日,同一组学生排成一长行。六月 33 日,同一组学生每行只站一人。六月 44 日,同一组学生每行 66 人。这个过程一直持续到六月 1212 日,每天每行人数都不同。然而到六月 1313 日,他们找不到新的排列方式。这组学生最少可能有多少人?
On June 1,1, a group of students is standing in rows, with 1515 students in each row. On June 2,2, the same group is standing with all of the students in one long row. On June 3,3, the same group is standing with just one student in each row. On June 4,4, the same group is standing with 66 students in each row. This process continues through June 1212 with a different number of students per row each day. However, on June 13,13, they cannot find a new way of organizing the students. What is the smallest possible number of students in the group?
2121
3030
6060
9090
10801080
答案:C
知识点:因数个数最小公倍数分类讨论
小提示:
不同的每行人数就是总人数的因数
The different row sizes are the divisors of the group size.
大提示:
需要最小的同时为 66 和 1515 的倍数,并且恰好有 1212 个因数
You need the smallest multiple of both 66 and 1515 with exactly 1212 divisors.
视频讲解:
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文字解答:
每行可站的人数正好是学生总数的正因数。因为六月 11 日至六月 1212 日有不同排列,而六月 1313 日没有新的排列方式,所以总人数必须恰好有 1212 个正因数。
总人数必须同时能被 1515 和 66 整除,因此能被 lcm⁡(15,6)=30=2⋅3⋅5\operatorname{lcm}(15,6)=30=2\cdot3\cdot5 整除。而这个数只有 88 个因数。
最小的 3030 的倍数且有 1212 个因数的是 60=22⋅3⋅560=2^2\cdot3\cdot5,其因数个数为 (2+1)(1+1)(1+1)=12(2+1)(1+1)(1+1)=12。
所以正确答案是 C。
The possible numbers of students per row are exactly the positive divisors of the total number of students. Since June 11 through June 1212 give different arrangements and June 1313 gives no new one, the total number of students must have exactly 1212 positive divisors.
The number must be divisible by both 1515 and 6,6, hence by lcm⁡(15,6)=30=2⋅3⋅5.\operatorname{lcm}(15,6)=30=2\cdot3\cdot5. This number has only 88 divisors.
The smallest multiple of 3030 with 1212 divisors is 60=22⋅3⋅5,60=2^2\cdot3\cdot5, which has (2+1)(1+1)(1+1)=12(2+1)(1+1)(1+1)=12 divisors.
Thus, C is the correct answer.
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