Of all the students in a certain dormitory, ~$1 \over 2$~are first-year students and the rest are second-year students. If ~$4 \over 5$~ 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?

~$1 \over 15$~

~$1 \over 5$~

~$4 \over 15$~

~$1 \over 3$~

~$2 \over 5$~

##### 考题讲解

 选专业 未选专业 总和 一年级学生 1/5*1/2*M 4/5*1/2*M 1/2M 二年级学生 3*1/5*1/2*M 2/5*1/2*M 1/2M 总和 2/5*M 3/5*M M