题干当中有很多比例的表达，容易理解错题目的意思。

例：Of the students in a school, 20 percent are in the science club and 30 percent are in the band.If 25 percent of the students in the school are in the band but are not in the science club, what percent of the students who are in the science club are not in the band?

A. 15%

B. 20%

C. 25%

D. 60%

E. 75%

这道题很容易选到A选项，假设science club是集合A，band是集合B，根据题意可得A=20%, B=30%, B-A∩B=25%, 所以A∩B=5%，那么A-A∩B=15%，但是问题不是所有人的百分之多少是属于A而不属于B的，而是问A的百分之多少不属于B，所以最终计算的比例应该是(A-A∩B)/A=15%/20%=75%，正确答案是E.

题目给的信息有一定的迷惑性，做题的时候容易自己脑补出额外的信息。

例：In a graduating class of 236 students, 142 took algebra and 121 took chemistry. What is the greatest possible number of students that could have taken both algebra and chemistry?

这道题很容易错算成27，就是把236当作两个集合的并集对待，但是题目只是说一共236个人，并没有说全部的人都一定会去学这两门学科，所以并集是一个未知数，题目问的交集的最大值，也是从侧面提示算出来的交集并不是一个确定的数字，只是一个范围，所以能用的信息其实只有algebra人数142，chemistry人数121，两者交集最大值就直接取比较小的那个集合即可，就是121.

有一些题目用对了方法解题会更快。

例：Of the 500 delegates attending a convention, 200 are Republicans and the rest are Democrats. One hundred of the delegates are vegetarians and, of those who are not vegetarians, 270 are Democrats. How many of the vegetarian delegates are Republicans?

A .30 

B .40  

C .50 

D .60 

E .70 

像这样把所有的人按照不同的方式进行分类的集合题，可以利用列表格的方式来解题，如图：