[讨论]10月math机经38题

以下是引用xuxu在2003-10-6 0:30:00的发言：
7/9 is correct.

here is what i thought about this:

first semester: 1/2 enrolled in biology, 1/3 chemistry, and the rest of the 1/6 enrolled in physics.  

second semester: the remaining 1/3 (1/2*2/3) enrolled in biology, the same thing applies to those who still enroll in chemistry, that is 1/4 (1/3*3/4). however, the # of student who choose physics remains the same.

therefore, the result should be: (1/3+1/4)/(1/3+1/4+1/6)=7/9

another way: you can always assume a particular number to cope with this type of percentage thing.  suppose there were 60 students in the first semester, then, it is pretty straitforward to get 30 on biology, 20 on chemistry, and the rest of 10 on physics. the second semester, things changed.  now, 20 are left with biology, 15 left with chemistry, and as to the physics, the # is still 10. then, it is easy to get the deal: 20+15/20+15+10=7/9.

in some cases, it is my feeling to come up with a figure to tackle this type of percentage thing, which just gives me the doulble assurance.