Of all the students in a certain dormitory, 1/2 are first-year students and the rest are second-year students. If 4/5 of the first-year students have not declared a major and if the fraction of second-year students who have declared a major is 3 times the fraction of first-year students who have declared a major, what fraction of all the students in the dormitory are second-year students who have not declared a major?

A. 1/15

B. 4/15

C. 1/3

D. 2/5

我有思路,但是不知道哪里出问题了,算出来和哪个选项都不一样.

你就根据它题目说的,一个条件一个条件的把它能得出得结论写下来.

first year 4/5 not then 1/5 yeah

second year 3/5 yeah then 2/5 not

first year and second year 各一半

所以 (2/5)/2=1/5

怎么算出个1/5来啊.我哪里有问题啊?情纠正.

可能你的题目有错误，我的这套里面答案b就是1/5。

其实一点一点的推就是了，主要是看细心不细心。

其实lugubrious表达得很简捷准确

但是这样可能好理解一些：设总人数是2a。则一年级二年级人数各为a.

引用一下lugubrious的表达：

first year 4/5 not then 1/5 yeah（注意：这里的4/5是部分的比例即：一年级内部比较，下面的也一样）

second year 3/5 yeah then 2/5 not

所以the number of second-year students who have not declared a major=2/5*a(2/5是二年级内部的比较所以人数是a不是2a) 即：2a/5

答案问的是：what fraction of all the students in the dormitory are second-year students who have not declared a major? (这里问的就是与总数的比较了)

因此应该是the number of second-year students who have not declared a major(2a/5) / the totle number of the students in the dormitory(2a) 即：(2a/5)/2a=1/5

我觉得这样是对的，里面容易弄错的就是比较的对象是部分还是整体的问题，

请指教！