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Xiaoshengchu mathematics competition questions
Suppose there are x people in the sixth grade.

Everyone bought two or three copies.

You can know the number of people who buy science and technology books+literature books+reference books-the number of people who buy all three books = 2 * X.

3/5*X+4/5*X+3/4*X-42=2X

X=280 people

Everyone bought two or three copies,

The number of people who buy science and technology books+literature books+reference books is how many books they buy. It is 2* the number of people who buy 2 copies +3* the number of people who buy 3 copies.