Idea: Because it was sold out in three days, it can be seen that 60%+240 copies were sold on the first day and the second day.

Selling 240+ on the third day is equal to 40% of the total!

The ratio of books sold on the second day to books sold on the third day is 9:5, so there are x books in total:

x=30%x+30%x+240+(30%x+240)*5/9

X= 1600 copies!