3、There are 100 students in the class. Each student must choose at least one course of English, French, and German. But no student can choose all the three class. There are 30 students learn E, and 60 students learn French, and 15 students learning German also learn English. How many students learn German? (students learning E do not learn French and F not G) 【答案】25 【思路】100-30-60+15=25，学E不学F，也就是E与F的交集为0，同理F与G的交集也为0，那么只有E与G有交集为15，则学E学G的人为60+30-15，现在要求F只要把学E学G的减掉即可。 由题知：EG交集是15 EF没有交集、FG没有交集。我感觉上面的思路好像描述错了，60+30-15 应该是学E和学F的，100-（60+30-15）是单独学G的，要想得出有多少学生学G，就应该是 单独学G的学生+学G又学E的学生 60+30-15+15 =35。不知道对不对啊。我是不是没有注意到什么呢？请大N批判。 | 
