138 students subscribed to the Junior Newspaper or the composition of primary school students, among which 5 of 6 students subscribed to the Junior Newspaper and 2 of 3 students subscribed to the composition of primary school students.
The standard answer is 57.5%, provided that all students have subscribed to the book, and there are no students who have not subscribed to the book. The biggest condition is that no student has subscribed to two books at the same time. The formula is: (80%+75%+60%-100%)/2 = 57.5%. Note: Add up the proportions of the three books (the number of people who have repeatedly ordered all the books has increased three times at this time), subtract 100% (the number of people who have repeatedly ordered all the books is exactly twice) and then divide by 2.
Why 60% is wrong is to assume that the total number of people is 100, the number of people who subscribe to the composition is 15 is 20, the number of people who subscribe to the three types is 60, 20+ 15+60=95, and five people have not subscribed, which does not meet the preconditions.
If the preconditions allow three kinds of students who don't order books, answer 60% directly. But this obviously does not conform to the teaching spirit.