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.